referring now to the drawings and more specifically to fig1 it will be seen that a conventional lens generating apparatus designated generally by the reference numeral 10 is thereshown . the lens generating apparatus includes a bottom slide 11 having a lens grinding tool assembly 12 rotatably mounted thereon . the lens grinding tool assembly 12 includes a table 13 having a motor and high speed spindle unit 14 thereon and being pivotally mounted by a vertical pivot 15 to the bottom slide 11 . it will be noted that the pivot connection is eccentric relative to the table to permit the table to swing in horizontal arcuate fashion about a vertical axis relative to the bottom slide . a double - acting hydraulic ram unit 16 is mounted on the bottom slide by a mounting plate 18 and the pivot rod thereof is pivotally connected by a pivot 17 to the table 13 to produce power shifting of the table about the pivot 15 . the output shaft of the motor and high speed spindle unit 14 is provided with a tool holder chuck 19 for holding the grinding tool used in the grinding operation . a lens collet assembly 20 is mounted on the tail stock slide 21 and includes an auxiliary slide 20a . it is pointed out that the entire collet assembly may be moved longitudinally of the bottom slide 11 . the drive system also permits advancement and retraction of the collet 22 . the tail stock slide 21 is shiftable with a manually operable actuating wheel 21a which is operatively connected to the collet 22 to permit manual advancing and retractive movement of the collet . it is pointed out that all of the aforementioned features of the lens generating apparatus are of conventional construction and do not , per se , form part of the present invention . the collet housing 20a has an elongate l - shaped guide structure 23 secured thereto , as best seen in fig1 . the guide structure includes a vertical element 24 which is rigidly secured to the tail stock slide housing and which has an elongate horizontal guide element 25 integrally formed therewith and projecting therefrom . the horizontal guide element 25 has an elongate slot 26 therein which opens outwardly . referring now to fig2 and 4 , it will be seen that the tool holder chuck 19 of the lens grinding tool assembly 12 is provided with a grinding tool 27 . the grinding tool 27 includes a generally cup - shaped grinding wheel 28 having an elongate shank 29 that is received within the tool holder chuck 19 . when the motor and spindle unit are engaged , the grinding tool 27 will also be revolved . it is also pointed out that , when the motor and high speed spindle unit are operated , the hydraulic ram is programmed to operate , thereby producing oscillation of the entire lens grinding tool assembly . the cup - shaped grinding wheel 21 includes a cylindrical portion 30 having an outer cylindrical surface 31 and an inner cylindrical surface 32 . the cup - shaped portion also includes a conical portion 33 which extends downwardly to the shank 29 . the cup - shaped grinding wheel 28 also includes an arcuate end surface 34 that is continuous with the outer and inner cylindrical surfaces thereof . it will also be noted that the cylindrical portion 30 has a plurality of circumferentially spaced apart axially extending slots 35 therein . the outer cylindrical surface 31 , the inner cylindrical surface 32 , the arcuate end surface 34 , and a part of the surface of the conical portion 33 are grit - plated with a plurality of diamond particles . in the embodiment shown , the grinding tool has a 31 / 2 inch diameter and the spacing of the various diamond particles 36 is extremely important in permitting dry grinding of plastic lenses made of cr - 39 or polycarbonate . in this regard , the outer cylindrical surface 31 and the grit - plated conical portion 33 together have an axial dimension of 25 millimeters . this excludes the small axial dimension of the arcuate end surface 34 . the 25 millimeter exterior surface of the cylindrical portion and conical portion is coated with a 20 grit - plating . the inner cylindrical surface 32 has an axial dimension of 12 millimeters and is coated with a 20 grit - plating . finally , the arcuate end surface 34 has a 1 / 8 inch radius and is coated with 30 grit - plating . it will be seen that the greatest surface area of the diamond grit - plating is 20 grit , but that the small arcuate end surface 34 has a 30 grit - plating . with this arrangement , the grinding tool produces a plurality of sharp single point removal of the swarf material from the lens blank during the grinding operation . this eliminates material buildup between the lens blank and the grinding tool , and any heat generated by this grinding action is quickly removed by the rapid removal of the swarf material . one of the improved features of the lens generating apparatus 10 is the particular construction and operation of the cylindrical chamber structure 37 , which includes a bottom wall 38 , a peripheral or circumferential wall 39 , and a top wall or cover plate 40 . the top wall 40 has a circumferential flange 41 integrally formed therewith and depending therefrom . the bottom wall 38 has a depending pin 42 integrally formed therewith , which is journaled in a bearing ( not shown ) on an l - shaped bracket 43 . the l - shaped bracket includes a horizontal leg 44 and a vertical leg 45 , the latter being secured to the lens grinding tool assembly 12 for movement therewith . the journal element on the bottom wall 38 of the chamber structure permits the chamber structure to remain in alignment during movement of the lens grinding tool assembly 12 . the chamber structure 37 has a small bracket 46 affixed thereto and projecting outwardly therefrom . the bracket 46 has a guide bolt 47 extending through a threaded opening in the bracket and engaging in the elongate slot 26 of the guide structure 23 . the guide bolt 47 prevents rotation of the chamber structure during oscillating movement of the lens grinding tool assembly 12 . the chamber structure 37 has an enlarged first opening 48 in the circumferential wall 39 thereof , and the edges defined by the opening 48 are covered with a yieldable cover element 49 . the collet 22 projects through the opening 48 and into the chamber structure 37 during the grinding operation . it will be noted that the cross - sectional size of the opening 48 is substantially larger than the cross - sectional size of the collet 22 so that a substantially large unobstructed zone is defined exteriorly of the collet . this unobstructed zone permits the continuous flow of high volume , high velocity ambient air into the chamber structure during the grinding operation . this air flow is essential in performing a cooling action and also permitting the swarf material to be entrained in an air stream and evacuated from the chamber . the circumferential wall 39 of the chamber structure 37 also has a second opening 50 therein and this opening extends circumferentially in an arc greater than 180 degrees . the grinding tool projects through this opening and the large size of the opening permits movement of the lens grinding tool assembly continuously through its laterally oscillating cycles . means are provided for limiting the size of the large second opening 50 to a size which permits the lens grinding tool and associated chuck to project through the housing , but which effectively closes the remaining portion of the opening 51 while allowing lateral oscillation of the lens grinding tool assembly 12 . this means includes a pair of elongate vertically disposed rubber curtains 51 , each being secured at one end thereof by one of a pair of clamping plates 52 secured to the peripheral wall 39 by bolts 53 adjacent the first opening 48 . each of the rubber curtains 51 has its other end portion wound upon a pair of spring loaded rollers 54 revolvably mounted on suitable brackets carried by the lens grinding tool assembly 12 . it will be appreciated that , as the lens grinding tool assembly 12 oscillates through a horizontal arc relative to the chamber structure 37 , one of the rubber curtains will be unwound from its associated spring loaded roller 54 , while the other curtain will be wound upon its roller . this arrangement effectively closes the large second opening 50 around the tool chuck housing of the lens grinding tool assembly 12 . the top plate of the chamber structure has an opening 55 therein which is provided with a fitting 56 . the fitting 56 has one end of an elongate bellows - type hose 57 secured thereto by a clamp 58 . the other end of the hose 57 is connected to a vacuum driven fan unit 59 including a fan housing 60a having a fan 60 mounted therein and driven by a suitable motor 61 . during the grinding operation , the swarf material will be evacuated through the vacuum hose 57 into the fan housing 60a and will eventually be directed to a collection box 62 . the vacuum driven fan unit 59 is provided with suitable filters ( not shown ) and baffles ( not shown ) to permit separation of the air from the particles of swarf material . the collector 62 may be provided with collection bags to facilitate disposal of the swarf material . the optical lenses which are generated are formed from plastic material , such as cr - 39 or polycarbonate . when polycarbonate is used , swarf sometimes tends to adhere to the irregular surface of the grinding tool 27 . means are therefore provided for eliminating this condition and this means includes a mist dispensing nozzle 63 which projects through an opening in second opening 50 and which is secured thereto by a clamping plate 64 . the mist dispensing nozzle is connected to a conduit 65 which has its other end connected to a source of liquid lubricant and air pressure . when the mist generator system is actuated , the mist dispensing nozzle 63 will direct an extremely fine mist into the zones defining the interfaces between the lens blank and the grinding tool . the mist generator includes an air line connected to a reservoir 67 containing the lubricant . an air line 67a forms the liquid lubricant into a mist . the mist prevents the adhesion of the swarf particle of polycarbonate with the grinding tool and these particles are entrained in a stream of air during the grinding operation and are evacuated through the vacuum hose 57 . in operation , the lens blank will be secured to the distal end of the lens collet 22 . the collet will be advanced until the collet is positioned within the chamber structure 37 and in contacting relation with the grinding tool 27 . it will be appreciated that the particular curves to be generated on the lens blank are typically pre - programmed into a computer controller which controls the operation of the collet and grinding tool assembly . during the grinding operation , the vacuum driven fan unit 59 will be energized so that , as the particles of swarf are continuously removed from the lens blank , high volume , high velocity ambient air will pass through the opening 48 and will entrain the swarf particles therein for passage through the vacuum hose 57 . the rapid removal of the particles prevents the formation of a mulch at the interface between the lens blank and the grinding tool . therefore , heat buildup , which is normally attendant in the conventional grinding operation because of the buildup of material at this interface , will be eliminated . further , the spacing between the diamond elements 36 forming the grit plate permits single point removal of the material , which reduces the friction buildup . the swarf material will accumulate in collection bags in the collector 62 and may be readily removed and disposed of . there will be no dust particles generated during the grinding operation because the high volume , high velocity ambient air passing through the opening 48 and the continuous vacuuming of the particles through the vacuum hose 57 simply prevents this from occurring . when polycarbonate lenses are generated , the mist is only sufficient to prevent adherence of the polycarbonate particles to the grinding wheel . the mist does not produce a wet interior of the chamber structure in the manner of wet grinding . the particles are quickly entrained in the air stream and are evacuated for disposal in the collector 62 . from the foregoing , it will be seen that i have provided a novel process and apparatus for dry grinding of lenses from plastic material , such as cr - 39 and polycarbonate . it will also be seen , from the preceding description , that the various components which permit dry grinding of plastic lenses may be easily retrofitted to any conventional lens generating apparatus . in this regard , the chamber structure 37 may be substituted for the conventional chamber structure . the vacuum removal and misting system will be provided , along with the novel grinding tool 27 . thus , it will be seen that i have provided a novel dry grinding process and apparatus which has never heretofore been practiced .