referring now in more detail and by reference characters to the drawings , and particularly to fig1 it can be seen that the ophthalmic pachymeter of the present invention comprises a television camera 20 having a conventional lens and which is aligned with and receives an image of the eye 22 of a subject through a beam splitter 24 for later quantification and for providing a television image of the eye for analysis . referring to fig3 it can be seen that the apparatus comprises a conventional incremental motor 26 for positioning an elongate aperture , e . g . a slit 28 , of a slit form 30 in the focal plane of light projector represented in fig1 by a lamp 32 . the slit form 30 may be operatively connected to a suitable slide assembly 31 , as also best illustrated in fig3 of the drawings . the slit form 30 , and particularly the slit 28 thereof , in conjunction with the lamp 32 , will produce an image at the eye 22 through the action of a projection lens 34 as shown in fig1 for selecting sequential images for analysis . referring now to fig6 there is illustrated a kohler arrangement 36 which comprises the lamp 32 having a dense filament 38 . in this case , by further reference to fig6 it can be seen that the image of the filament 38 is formed at the entrance pupil of the projection lens 34 by means of a condenser lens 40 . adjacent to the condenser lens 40 and in optical alignment with the condenser lens 40 is the slit form 30 carrying the optical slit 28 . this slit form 30 is preferably mounted on a carrier ( not shown in detail ). the carrier is in a direction perpendicular to the slit 28 by the aforesaid incremental motor 26 . the images of the slits are brought into focus in the same plane as the television camera 20 by the projection lens 34 and a system of mirrors and prisms including a regular mirror 42 and a dichroic mirror 42 &# 39 ;, ( both being interchangeable ) as shown in fig1 , 4 and 5 . a fixation lamp 44 , sometimes referred to as a &# 34 ; target lamp &# 34 ; or &# 34 ; fixation target lamp &# 34 ;, as shown in fig1 is also provided for operation with the beam splitter 24 , as shown . this combination of the lamp 32 , lenses 34 and 40 and mirrors and prisms 42 , as well as the slit form 30 , function as a slit lamp projector . referring again to fig1 it can be seen that the television camera 20 generates a signal representative of the image of the eye which is transmitted to a video amplifier 46 for amplification and mixing video signals for analysis . a flash analog - to - digital converter 48 receives the output of the video amplifier 46 for processing and digitizing analog signals received from the television camera 20 . a data buffer 50 receives an output from the analog - to - digital converter 48 for directing the digital data to and from a storage in the for of a digital data memory 52 . for example , the data buffer 50 and the data memory 52 may form part of a conventional computer which is not illustrated in detail herein . in this respect , it can be observed that many of the components are shown in schematic form ( rectangular boxes ) in fig1 . the digital data which is directed to the digital data memory 52 constitutes a storage of the numerical brightness of each element within the fiducial boundary . an address counter 54 is provided for determining the location and storage of the pixel brightness data for each pixel in the image . a mode controller 56 is connected to the address counter 54 and is provided for determining the sequence of operations of the system . the mode controller 56 receives an input from a computer interface 58 which , in turn is connected to the data buffer 50 and is also connected to the analog - to - digital converter 48 , as illustrated in fig1 . the computer interface 58 , operating in conjunction with a computer , controls the system elements through the associated computer . in this case , a display driver 60 is provided for controlling the aforesaid fixation target lamp 44 visible by reflection by the beam splitter 24 . this serves to render the apparent location of the fixation target lamp 44 , namely from the beam splitter . 24 coincident with the optical center of the television camera 20 and its associated lens system . a pulse shaper 71 receives an output from the computer interface 58 and provides a control to the display driver 60 and the camera 20 . turning now to fig1 , it can be observed that there is a representative image of the eye 22 . the slit beam 28 illuminated anatomical features are visible as a tyndall image 62 representing those portions of the eye , such as the corneal epithelium , the stroma and the endothelial layer which scatter the light . an iris 64 in the eye is not the area to be measured and therefore , the illumination of this area is an artefact of tyndall illumination . the iris image 64 may further be diminished by limiting the spectral distribution of the slit beam 28 through the use of a color filter ( not shown ). in addition , a slit projector , or slit projectors if more than one is used , produce specular reflections 66 , as shown in fig1 a , and which are located in &# 34 ; x , y &# 34 ; coordinate space , depending upon the surface curvature of the cornea of the eye 22 . the associated computer , through the action of the computer interface 58 and the mode controller 56 and display driver 60 ( see fig1 ), restrict the sample data to the corneal section 62 of the eye made visible by slit illumination . the iris 64 may be dilated maximally to provide a uniform dark background for the optical section and the slit height , which is restricted to eliminate bright reflections above and below the area of interest . the operator of the ophthalmic pachymeter is provided with a computer - generated figure which is used as a fiducial mark system illustrated by reference number 68 in fig1 . the fiducial marks in this fiducial mark system 68 are located around the center of the display monitor . the figure is preferably software - controlled to coincide with operation of the incremental or &# 34 ; stepper &# 34 ; motor 26 which moves the slit form 30 and hence , a half slit 160 also in this form 30 . thus , the computer generated fiducial marks are designed to be coincident with the slit position so that the operator is provided with a focus and alignment aid . referring now to fig4 and 5 , it can be seen that the slit lamp illuminator components , e . g . the lamp 32 and slit form 30 , as well as the television camera system 20 are mounted on a moveable base 70 which comprises a frame casting . a vertical positioning element , in the nature of a vertically arranged support shaft 72 is operative mounted on the base casting 70 , as illustrated . generally axles 74 which support toothed wheels ( not shown ) are located in the base casting 70 , as best shown in fig5 for motion toward and away from the subject . the device also comprises dust covers 78 which cover the toothed wheels . a friction creating member 76 is operated by a lever or handle 82 against the table surface 80 to cause the instrument to be moved by the operator for focusing and alignment . this arrangement allows for motion toward and away from the subject , as indicated . the base casting 70 is provided with internal bearings ( not shown ) to permit the assembly to move transversely , that is perpendicular to the forward and backward motion parallel to the optical axis of the instrument . the toothed wheels 76 , located under the dust covers 78 , serve to constrain the motion relative to a table 80 and hence , the patient so that movement occurs only in a specified area . the vertical positioning element , such as the support shaft 72 , raises and lowers the instrument relative to the subject to permit centering of the image in the television picture . the subject is positioned at the table 80 with a table - mounted chin and brow rest of conventional design for positioning and stabilizing the head during the measurement . inasmuch as this chin and brow rest is of a conventional construction , it is neither illustrated nor described in any further detail herein . however , the base casting 70 is provided with the upstanding handle 82 for manual manipulation by an operator of the apparatus to enable positioning of the instrument with respect to a subject and which is also hereinafter described in more detail . the beam splitter 24 may be mounted on the base plate 84 of a housing 86 which houses many of the components of the ophthalmic pachymeter , such as , for example , the television camera 20 the condenser lenses 40 , the slit form 30 , the lamp 32 and the mirrors and / or prisms 42 . located beneath the beam splitter 24 is a printed circuit assembly ( not shown ). this printed circuit assembly may contain the fixation lamp 44 . otherwise , the fixation lamp 44 may be mounted above the beam splitter 24 in the manner as best illustrated in fig4 and 5 of the drawings . this beam splitter 24 and the fixation target lamp 44 provide a bright target for determining the point of gaze for the subject . the brightness of this target may be controlled to permit persons with low visual acuity to perceive it and to fixate upon it . in a more preferred embodiment of the invention , the fixation lamp 44 is preferably a light - emitting diode - type lamp and is preferably bi - colored with pulse drive to present a visible pulse stream of alternative colors at about a one second interval rate . the use of this type of fixation lamp 44 and the associated drive provides a wide range of brightness so that the target can be fixated upon by the subject irrespective of visual acuity of the subject . the co - axial location of the fixation target assures maximal ability to accurately reconstruct the three dimensional data . the fixation lamp 44 , which causes the iris 64 and the sclera to be illuminated , not only provide for an image of the eye , but also enable an image to be generated for record - keeping purposes . the normal illumination levels , when slit images are being recorded , is usually inadequate to cause surrounding tissue to be well defined for overall viewing . the common slit lamp camera uses the optical system of the bio - microscope and due to the length of the focal ratio of these systems . a large amount of flash energy is required for exposures . the present invention , however , provides a much more efficient optical design and thus , the flash energy is reduced by orders of magnitude , when compared to conventional slit lamp photography . the reduction of light energy entering into the eye is , of course , a desirable feature for subject safety and comfort , and also ensures more reliable data . the operator of the ophthalmic pachymeter will position the half slit images 62 , as shown in fig1 a , into coincidence so that , in effect , the two half - slit images form somewhat of an &# 34 ; s &# 34 ; shape , as shown in fig1 b . this will occur with reference to the fiducial figure 68 of fig1 , which is displayed for the operator , to thereby align and thereby focus the instrument . the half - slits , as shown , are effectively positioned by the computer in the optical center line of each projector . the operator moves the instrument , preferably by manual manipulation of the handle 82 in order to obtain this coincidence , as hereinafter described , in order to form this s - type image arrangement . when the s - type tyndall image has been formed of the half tyndall images , the operator may then take the necessary data . the motors 26 which move the slit form 30 will slew the full length slits 28 of fig3 across the eye from each side sequentially to provide the data sequence which will ultimately be stored for analysis . the data is masked by software in order to eliminate extraneous material . the arc of the tyndall images lies on only one side of the iris section illuminated by the light which is passed through the cornea and has a definable maximum number of pixel loci at the apex from the iris line . the area of this pixel loci is defined by software within the system for each frame and only the data which falls within this defined area is stored for analysis . as a possible exception , a small area at the center which contains the reflection of the fixation lamp may also be stored for compensation of involuntary movements of the eye . as indicated previously , the operator of the pachymeter can position the half - slits images 62 . this can be accomplised by manual manipulation of the handle 82 in order to position the television camera 20 in three dimensional space relative to the eye . the desired alignment is obtained by viewing the display before recordation of the data to be analyzed . the generation and positioning of the box , circle or other limiting fiducial marking is by well known computer techniques that are not detailed herein . the operator simply adjusts the controls so that the optical sections coincide at the center of the display monitor . this action assures the operator that the focus and area being measured are correct . the focus and image location are simultaneously adjusted by the operator with reference to the display that shows the image from the camera with the fiducial markings superimposed . a picture formation of a tyndall image 62 is generated in the television camera 20 . a given point on the tyndall image 62 is projected onto the photo - sensitive area of the television camera 20 . the datum of this given point on the tyndall image , after an analog - to - digital conversion , represents an x , y locus with associated brightness . the slit 28 positioned under computer control by the incremental motor 26 is at a known location relative to the optical centerline of the camera 20 . the projector optical axis relating to the camera axis is established in manufacture at a known angular relationship . since that angle is known , the magnification is known and that the slit position is also known , the angle φ is thereby defined in the associated computer software . the height of the datum above the reference plane δh is then calculated . each raster line intersection with the tyndall image 62 is used to calculate the associated height value . after the series of images which comprise a complete measurement are so defined and stored in the computer memory , the surface contour for both surfaces of the cornea and the local thickness are displayed for use . the image which is generated may be identified as either a left eye image or a right eye image by means of a switch ( not shown ) and which can be located in the instrument base and which is also interfaced to the computer . with this identification , the location of the cursor in the fiducial image 68 is determined in the computer software . a transducer ( not shown ) may be utilized to provide a signal representative of instrument lateral displacement and is interpreted to determine the eye being examined , due to the fact that the slit lamp 32 is always displaced in the temporal direction for use . in the preferred embodiment , a point at the vertical center of the cursor in the fiducial fig6 , displaced a few pixels toward one side , is identified in the software and can serve as a sample for black clamping of the video signal and which is usually accomplished by conventional circuitry . the image of the cornea is located and stored by computer software , based on known characteristics of the corneal image . all initial pixel values for the enclosed line segments of the corneal image , so identified , are averaged for reflected light intensity in terms of pixel brightness and the resultant numerical constant is used to determine the optical character of the remainder of the tyndall image 62 . after determination of the corneal pixel loci , the corneal thickness is derived by known magnification projection angle , surface shape and pixel pitch . the data are then stored by location in an area of the memory for later use . referring again to fig6 which indicates a kohler projector , it can be seen that the image of the slits are brought into focus in the same plane as the television camera 20 by the projection lens 34 and the system of mirrors or prisms 42 , as previously described . the beam path is folded by the mirrors or prisms 42 in order to achieve compact assembly . the focal length of the projection lenses 34 is made to be as long as possible to reduce beam convergence or divergence at the eye which would otherwise degrade the tyndall image 62 . in general , the projection lens 34 is selected to provide an aperture sine function on the order of 0 . 05 or less for best results . the aperture sine is calculated from the optical components by the formula ; f / d 2 where f is the focal ratio of the lens and d is the distance from the slit to the exit pupil . the brightness of the slit image e is calculated by the formula , e =( f / d 2 ) db , where d is the optical transmission factor for the lens and b is the luminance of the filament source , e . g . the filament 38 . the use of aspheric condenser lenses , optical coatings for all surfaces and a low ratio beam splitter for the fixation target permit the use of lamps in the range of 20 watts that provide over . 400 lumens as the light source . the minimum brightness level of the slit image reflection is dependent upon the sensitivity of the camera employed . the reflected light is on the order of 4 % or less of the incident light and the greater the illumination level of the diffuse reflection , the better the signal to noise ratio of the resultant television signal . the use of halogen cycle lamps improves the stability of light output with time and provides the best available lamp design . in addition one or more optical filters 90 located on a support pivoted to the housing 86 by a pivot pin 92 and positionable by a handle 94 ( fig5 ), or computer controlled mechanism ( not illustrated ), are included in the illumination path for selected illumination wave band determination . the optical filter 90 also serves to limit energy delivered to the eye 22 to reduce the possibility of photo - toxic reaction hazard to the subject . the optical filter 90 for this purpose has little or no ultra - violet or infra - red transparency . the television display is in the form of a raster as shown in fig7 . the television has a monitor 100 which displays the visual information in time sequence . the beam current is low for black areas 102 and high for white areas 104 and scaled in magnitude to recreate the brightness range of the original scene . the television camera 20 generates the voltage analog of scene illumination that is provided with synchronizing signals to assure that the time sequence as reproduced is a faithful recreation of the scene being photographed . the following section more specifically describes the process employed in determining thickness and topography of the cornea . however , and while the circuitry as shown in fig8 literally constitutes a part of the apparatus , it is nevertheless described in connection with this image processing and operation , since it is integrally related to the image processing and operation . in fig1 , the relationship between the tyndall image 62 and the topography of the cornea is shown . along each raster line in the television display , there is a detectable edge of the tyndall image which has a virtual image location displaced by delta d ( δd ). this displacement distance is from the point at which the beam would have intersected the optical axis , if undeflected , as best shown in fig1 . from this image pixel locus the height of the datum above the reference plane , delta h ( δh ), can be calculated . the calculations are performed for all intercepts in all data frames to provide a matrix of x coordinate loci from which the topography can be plotted . referring now to fig9 the voltage waveform produced by the television camera of the pachymeter , is illustrated . as indicated previously , the beam is low for black areas 102 and high for brighter areas 104 . the brightness amplitude ratio of the anterior edge of the corneal section to the dark pupillary area representing the anterior chamber is used as a reference value for lens reflection assessment . the pixel amplitudes for all elements of the reference areas are averaged to provide the baseline reflectance value . the television signal voltage wave form , as shown in fig9 is a single raster line of video information in which there are bright areas 108 from the image of the cornea and a brighter image of the iris 64 ( represented by the bright areas 104 ) illuminated by the slit beam after the latter passes through the cornea . a sync pulse signal 112 precedes each line of pictorial information carrying voltage levels . after the sync pulse 112 a short period of a low level blanking pulses 114 follows . the blanking pulse 114 insures that the display is off while the beam is retraced to the start of a new line . the black level , represented by reference numeral 116 , is the most negative of the pictorial data voltages in the video composite signal . this level is determined by a keyed clamp circuit of conventional design where a selected spot in the image representing the anterior chamber signal is sampled and used as a minimum brightness determinant . as the voltage increases , the brightness also increases in the displayed image from black to peak white 118 representing saturation of the signal . the voltage level produced at saturation by a &# 34 ; white &# 34 ; image 118 is shown by the dotted line at the top of the illustration . the brightness profile of the corneal image will vary as the local optical density and index of refraction varies . at the leading edge in time of the corneal reflex signal , the signal rises to a peak 119 which represents the cornea to air interface . the amplitude of this signal is quite constant from subject to subject and from time to time . this constant interface signal is used for signal reference against which reflex measurements are made to quantify corneal transparency . each succeeding raster line will then provide a density profile for a different portion of the cornea . in the preferred embodiment of the present invention , the optical slit form 30 is moved in small lateral increments by the incremental motor 26 for sequential data sampling . in an alternative embodiment , the optical slit for form , 30 and incremental motor 26 are replaced by rhonchi rulings of suitable pattern dimension to provide several parallel slit beams in a single exposure . the plural beam system reduces the time required for data acquisition but complicates the computer processing of the data from the tyndall images 62 . in a further alternative embodiment , the slit 28 can be replaced by a liquid crystal display element so structured as to form electronically selected transparent areas substantially equivalent to the various slit positions in the preferred embodiment of this invention . each exposure containing the tyndall image or images 62 , is converted to digital form by the analog to digital converter 48 . through the action of the data buffer 50 , the mode controller 56 the address counter 54 and the digital data memory 52 these sequential amplitude values are stored for use . the data in storage represents the pixel brightness versus locus for each slice of the cornea to be analyzed . each successive pixel of each successive frame is then multiplied by a constant derived from the cornea to air interface signal average and the optical constant that corrects for the lower normal brightness . as each point is calculated , it is returned to storage in the same sequence for later computation and display . tyndall illumination provides three - dimensional data sequences of data that are transferred to the computer by the action of the computer interface 58 . in an area scan using the apparatus of the present invention , the processing time is so small compared to the user &# 39 ; s ability to resolve time that the actual imaging takes place on a real time basis . in other words , the analysis and determinations , e . g ., optical thickness , are made on a real time basis . fig8 represents a schematic diagram of part of the electronic circuitry employed in the preferred embodiment of the present invention . the composite video signal from the television camera 20 is applied to the input 120 of a signal conditioning amplifier . the terminated signal is buffered by an emitter follower 122 which drives dc restoration and sync stripper networks 124 and 126 . the dc restored and limited video is buffered by a second emitter follower and serves to drive clamping and mixing amplifiers 128 and 130 . a computer derived black reference timing signal 132 is generated in temporal synchronism with the area of the picture from the television camera 20 which defines the pupillary area near the center of the picture . this pulse is conditioned by mono - stable circuits 134 to provide a constant amplitude and constant width sampling pulse . this sampling pulse , via a capacitor 136 allows the capacitor to store a voltage sample of the raw video that represents the &# 34 ; black &# 34 ; level . the black reference level thus generated biases the amplifier 130 for use in the analog to digital converter 48 . the signals representative of reference levels are received at output nodes 137 and 138 , as shown in fig8 . signals from the computer are used for regeneration of the television timing in a conventional integrated circuit device 140 which makes use of a composite sync signal 142 and dot clock signal 144 from the computer display driver . the computer generated fiducial signal 146 and regenerated composite sync are mixed via resistors 148 with the video signal from an emitter follower 150 for providing the monitor signal . the monitor signal is used to drive a conventional crt display for use as a viewfinder by the user of the pachymeter of the present invention . a potentiometer 242 is provided for setting sync injection amplitude to conform to ire or smpte standards . the display of the data can take the form of a single frame &# 39 ; s information that can be displayed as false colored areas for relative transparency , for example . the entire set of frames may be combined to form a virtual three - dimensional display of surface contour or membrane thickness as needed . the data also may be presented simply as a numerical value for average optical density , density area or other forms that the user finds most useful , by the use of well - known display techniques . in most , if not all cases , the exact optical density profile is of less interest to the clinician than the shape and thickness . for example , in refractive surgery , the incision depth should be at least ninety percent of the local corneal thickness without total penetration . when a suitable image or sequence of images has been stored and the requisite computations performed in the computer , the digital information that defines the cornea can be displayed in some arbitrary color upon the monitor together with the alphanumeric information image from computer by conventional video mixer means . alternatively the data may be presented for use in any of several formats such as plotted graphs , tabular numerical form , pseudo three dimensional shaded surface plots or other formats that are well known in the art . referring again fig3 the motion imparted to the slit form or so - called slit carrier member is controlled by the computer through the action of the incremental stepper motor 26 . in an alternative embodiment a second slit 160 of lesser length than the slit 28 is provided in both beam projection paths for the purpose of focusing the instrument . the two half slits so produced are placed by the operator into contact at the point of reflection of the fixation lamp 44 to establish proper alignment prior to recordation of the image sequence . thus , there has been illustrated and described a unique and novel ophthalmic pachymeter which enables determination of the thickness and relative optical density of the cornea on a real - time basis and which thereby fulfills all of the objects and advantages which have been sought . it should be understood that many changes , modifications , variations and other uses and applications will become apparent to those skilled in the art after considering this specification and the accompanying drawings . therefore , any and all such changes , modifications , variations and other uses and applications which do not depart from the spirit and scope of the invention , are deemed to be covered by the invention .