in the following , embodiments of the present invention will be explained with reference to the accompanying drawings . fig1 shows , as representative of the embodiments of the present invention , the basic lens configuration of examples 1 and 2 , whereas fig2 shows the schematic configuration of an image readout apparatus using the image readout lens in accordance with the present invention . as shown in fig2 the image readout lens 1 in accordance with the present invention is an imaging lens used in an optical system of an image readout apparatus 2 such as facsimile machine or image scanner . in this image readout apparatus 2 , the image readout lens 1 is disposed between a glass plate 4 for mounting an original 3 and a ccd cover glass 6 of a linear ccd 5 , and an illumination device 7 is disposed on the image readout lens 1 side of the glass plate 4 . in this image readout apparatus 2 , as the illumination device 7 emits light toward the original 3 , a luminous flux reflected by the original 3 forms an image via the image readout lens 1 , and thus formed image is read out by the linear ccd 5 . as shown in fig1 the image readout lens in accordance with examples 1 and 2 is constituted by four lenses l 1 to l 4 , and a stop i disposed between the second lens l 2 and the third lens l 3 , whereby the luminous flux incident thereon from the object side along its optical axis x forms an image at the imaging position p . this image readout lens comprises , successively from the object side , the first lens l 1 made of a positive meniscus lens which is convex toward the object , the second lens l 2 and the third lens l 3 each made of a negative lens having an image - side surface concave toward the image , and the fourth lens l 4 made of a biconvex lens . also , a lens glass material of at least one of the second lens l 2 and third lens l 3 satisfies the following conditional expression ( 1 ): θ g · f is the relative partial dispersion of the lens glass material defined by θ g · f =( n g − n f )/( n f − n c ); and υ d is the abbe number of the lens glass material defined by υ d =( n d − 1 )/( n f − n c ) n g is the refractive index of the glass material at a wavelength of 435 . 8 nm ; n f is the refractive index of the glass material at a wavelength of 486 . 1 nm ; n c is the refractive index of the glass material at a wavelength of 656 . 3 nm ; and n d is the refractive index of the glass material at a wavelength of 587 . 6 nm . further , this image readout lens satisfies the following conditional expressions ( 2 ) to ( 4 ): f 1 is the focal length of the first lens l 1 ; f 3 is the focal length of the third lens l 3 ; f 4 is the focal length of the fourth lens l 4 ; and f 23 is the composite focal length of the second lens l 2 and third lens l 3 . when the glass material of at least one of the second lens l 2 and third lens l 3 satisfies the above - mentioned conditional expression ( 1 ), chromatic aberration can favorably be corrected , whereby the image readout lens can become a lens suitable for reading out originals with color images . above the upper limit of conditional expression ( 1 ), axial chromatic aberration cannot favorably be corrected , whereby the focal positions for the respective colors may shift from each other along the optical axis direction , thus failing to yield a focal depth necessary for reading out color images . as a lens glass material having the relative partial dispersion θ g · f and abbe number υ d satisfying conditional expression ( 1 ), a glass material having anomalous dispersion , for example , can be used . a color - image readout lens with higher performances can be obtained when conditional expressions ( 2 ) to ( 4 ) are further satisfied . above the upper limit of conditional expression ( 2 ), astigmatism would increase , and distortion may not favorably be corrected . below the lower limit thereof , image surface curvature would increase , whereby homogenous imaging performances may not be obtained throughout the plane . above the upper limit of conditional expression ( 3 ), coma may not favorably be corrected . below the lower limit thereof , spherical aberration and image surface curvature may not appropriately be corrected , whereby homogenous imaging performances may not be obtained throughout the plane , either . conditional expression ( 4 ) defines the ratio of the composite focal length of the second lens l 2 and third lens l 3 to the focal length of the fourth lens l 4 . above the upper limit thereof , image surface curvature may not favorably be corrected . below the lower limit thereof , both astigmatism and distortion would increase , whereby imaging picture quality may deteriorate . in the following , examples 1 to 5 will be explained with reference to specific values . the image readout lenses in accordance with examples 1 to 5 are normalized at a focal length of 100 mm , and can be used with their focal lengths being determined for the respective sizes of originals as each example is proportionally reduced or enlarged in conformity to the size of the original to be read out . as shown in fig1 the image readout lens in accordance with example 1 comprises , successively from the object side , the first lens l 1 made of a positive meniscus lens having a convex surface directed onto the object side , the second lens l 2 made of a biconcave lens having a surface with a larger curvature directed onto the image side , the third lens l 3 made of a negative meniscus lens having a convex surface directed onto the object side , and the fourth lens l 4 made of a biconvex lens having a surface with a larger curvature directed onto the object side , in which the third lens l 3 and the fourth lens l 4 form a cemented lens . the following table 1 shows the radius of curvature r ( mm ) of each lens surface , center thickness of each lens and air space between neighboring lenses d ( mm ), refractive index n d and abbe number υ d of each lens at d - line ( wavelength : 587 . 6 nm ), and glass material product name of each lens in example 1 . in table 1 , as well as tables 4 , 6 , 8 , and 10 which will be mentioned later , numerals referring to the letters r , d , n d , and υ d successively increase from the object side . in table 1 , as well as tables 4 , 6 , 8 , and 10 which will be mentioned later , the lens in which “*” is attached to the left side of its glass material product name is formed from a glass material satisfying conditional expression ( 1 ). table 2 ( follows ) shows the refractive indexes n g , n f , and n c at the respective wavelengths of 435 . 8 nm , 486 . 1 nm , and 656 . 3 nm and relative partial dispersion θ g · f of the glass material for the third lens l 3 satisfying conditional expression ( 1 ) in example 1 . table 3 ( follows ) shows the values of composite focal length f , f number , imaging magnification β , and half angle of view ω of the whole lens system in the image readout lens in example 1 , and the values corresponding to conditional expressions ( 1 ) to ( 4 ) in example 1 . as can be seen from table 3 , conditional expressions ( 1 ) to ( 4 ) are satisfied in example 1 . fig3 a , 3 b and 3 c are aberration charts ( showing spherical aberration , astigmatism , and distortion ) of the image readout lens in accordance with example 1 , whereas fig4 is an aberration chart showing coma thereof . in these aberration charts , ω indicates the half angle of view . the spherical aberration chart indicates the respective aberrations with respect to e - line , g - line , and c - line . the astigmatism chart shows the respective aberrations with respect to the sagittal image surface ( s ) and tangential image surface ( t ). ( the same holds true for the following aberration charts , i . e ., fig5 , 8 , 9 , and 11 to 14 .) each of the aberration charts of the image readout lens in accordance with example 1 shown in fig3 and 4 refers to the state where glass plates ( refractive index : 1 . 52 ) having thicknesses of 6 . 48 mm and 1 . 51 mm are inserted in the optical path on the object side and image side , respectively . as can be seen from fig3 and 4 , all the kinds of aberration mentioned above can be made favorable in example 1 . also , since the respective aberration curves at the wavelengths of e - line , g - line , and c - line do not fluctuate much among them , it can be seen that chromatic aberration can favorably be corrected . when example 1 is used as being proportionally reduced for reading out the shorter side of a4 - size sheet , its focal length becomes 46 . 3 mm . the configuration of the image readout lens in accordance with example 2 is substantially the same as that of example 1 . the following table 4 shows the radius of curvature r ( mm ) of each lens surface , center thickness of each lens and air space between neighboring lenses d ( mm ), refractive index n d and abbe number υ d of each lens at d - line , and glass material product name of each lens in example 2 . as shown in table 4 , the third lens l 3 is formed from a glass material satisfying conditional expression ( 1 ). the glass material for the third lens l 3 is the same as that of example 1 . table 5 ( follows ) shows the values of composite focal length f , f number , imaging magnification β , and half angle of view ω of the whole lens system in the image readout lens in example 2 , and the values corresponding to conditional expressions ( 1 ) to ( 4 ) in example 2 . as can be seen from table 5 , conditional expressions ( 1 ) to ( 4 ) are satisfied in example 2 . fig5 a , 5 b and 5 c are aberration charts ( showing spherical aberration , astigmatism , and distortion ) of the image readout lens in accordance with example 2 , whereas fig6 is an aberration chart showing coma thereof . each of the aberration charts of the image readout lens in accordance with example 2 shown in fig5 and 6 refers to the state where glass plates ( refractive index : 1 . 52 ) having thicknesses of 6 . 49 mm and 1 . 52 mm are inserted in the optical path on the object side and image side , respectively . as can be seen from fig5 and 6 , all the kinds of aberration mentioned above such as chromatic aberration can be made favorable in example 2 as with example 1 . when example 2 is used as being proportionally reduced for reading out the shorter side of a4 - size sheet , its focal length becomes 46 . 2 mm . as shown in fig7 the image readout lens in accordance with example 3 comprises , successively from the object side , a first lens l 1 made of a positive meniscus lens having a convex surface directed onto the object side , a second lens l 2 made of a biconcave lens having a surface with a larger curvature directed onto the image side , a third lens l 3 made of a negative meniscus lens having a convex surface directed onto the object side , and a fourth lens l 4 made of a biconvex lens having a surface with a larger curvature directed onto the object side . the following table 6 shows the radius of curvature r ( mm ) of each lens surface , center thickness of each lens and air space between neighboring lenses d ( mm ), refractive index n d and abbe number υ d of each lens at d - line , and glass material product name of each lens in example 3 . as shown in table 6 , the second lens l 2 is formed from a glass material satisfying conditional expression ( 1 ). the glass material for the second lens l 2 is the same as the glass material for the third lens l 3 in example 1 . table 7 ( follows ) shows the values of composite focal length f , f number , imaging magnification β , and half angle of view ω of the whole lens system in the image readout lens in example 3 , and the values corresponding to conditional expressions ( 1 ) to ( 4 ) in example 3 . as can be seen from the table 7 , conditional expressions ( 1 ) to ( 4 ) are satisfied in example 3 . fig8 a , 8 b and 8 c are aberration charts ( showing spherical aberration , astigmatism , and distortion ) of the image readout lens in accordance with example 3 , whereas fig9 is an aberration chart showing coma thereof . each of the aberration charts of the image readout lens in accordance with example 3 shown in fig8 and 9 refers to the state where glass plates ( refractive index : 1 . 52 ) having thicknesses of 6 . 47 mm and 1 . 51 mm are inserted in the optical path on the object side and image side , respectively . as can be seen from fig8 and 9 , all the kinds of aberration mentioned above such as chromatic aberration can be made favorable in example 3 as with example 1 . when example 3 is used as being proportionally reduced for reading out the shorter side of a4 - size sheet , its focal length becomes 46 . 4 mm . as shown in fig1 , the image readout lens in accordance with example 4 is configured substantially the same as that of example 3 except that the stop i is disposed between the third lens l 3 and the fourth lens l 4 . the following table 8 shows the radius of curvature r ( mm ) of each lens surface , center thickness of each lens and air space between neighboring lenses d ( mm ), refractive index n d and abbe number υ d of each lens at d - line , and glass material product name of each lens in example 4 . as shown in table 8 , the second lens l 2 is formed from a glass material satisfying conditional expression ( 1 ). the glass material for the second lens l 2 is the same as the glass material for the third lens l 3 in example 1 . table 9 ( follows ) shows the values of composite focal length f , f number , imaging magnification β , and half angle of view ω of the whole lens system in the image readout lens in example 4 , and the values corresponding to conditional expressions ( 1 ) to ( 4 ) in example 4 . as can be seen from the table 9 , conditional expressions ( 1 ) to ( 4 ) are satisfied in example 4 . fig1 a , 11 b and 11 c are aberration charts ( showing spherical aberration , astigmatism , and distortion ) of the image readout lens in accordance with example 4 , whereas fig1 is an aberration chart showing coma thereof . each of the aberration charts of the image readout lens in accordance with example 4 shown in fig1 and 12 refers to the state where glass plates ( refractive index : 1 . 52 ) having thicknesses of 6 . 47 mm and 1 . 51 mm are inserted in the optical path on the object side and image side , respectively . as can be seen from fig1 and 12 , all the kinds of aberration mentioned above such as chromatic aberration can be made favorable in example 4 as with example 1 . when example 4 is used as being proportionally reduced for reading out the shorter side of a4 - size sheet , its focal length becomes 46 . 4 mm . the image readout lens in accordance with example 5 is configured substantially similar to that of example 4 as shown in fig1 . the following table 10 shows the radius of curvature r ( mm ) of each lens surface , center thickness of each lens and air space between neighboring lenses d ( mm ), refractive index n d and abbe number υ d of each lens at d - line , and glass material product name of each lens in example 5 . as shown in table 10 , the second lens l 2 is formed from a glass material satisfying conditional expression ( 1 ). table 11 ( follows ) shows the refractive indexes n g , n f , and n c at the respective wavelengths of 435 . 8 nm , 486 . 1 nm , and 656 . 3 nm and relative partial dispersion θ g · f of the glass material for the second lens l 2 satisfying conditional expression ( 1 ) in example 5 . table 12 ( follows ) shows the values of composite focal length f , f number , imaging magnification : βand half angle of view ω of the whole lens system in the image readout lens in example 5 , and the values corresponding to conditional expressions ( 1 ) to ( 4 ) in example 5 . as can be seen from table 12 , conditional expressions ( 1 ) to ( 4 ) are satisfied in example 5 . fig1 a , 13 b and 13 c are aberration charts ( showing spherical aberration , astigmatism , and distortion ) of the image readout lens in accordance with example 5 , whereas fig1 is an aberration chart showing coma thereof . each of the aberration charts of the image readout lens in accordance with example 5 shown in fig1 and 14 refers to the state where glass plates ( refractive index : 1 . 52 ) having thicknesses of 6 . 47 mm and 1 . 51 mm are inserted in the optical path on the object side and image side , respectively . as can be seen from fig1 and 14 , all the kinds of aberration mentioned above such as chromatic aberration can be made favorable in example 5 as with example 1 . when example 5 is used as being proportionally reduced for reading out the shorter side of a4 - size sheet , its focal length becomes 46 . 4 mm . without being restricted to the foregoing examples , the imaging lens in accordance with the present invention can be modified in various manners . for example , the radius of curvature r of each lens and lens spacing ( or lens thickness ) d can be changed as appropriate . as explained in the foregoing , the image readout lens in accordance with the present invention can achieve sufficiently favorable optical performances also as a color - image readout lens by defining the glass material of at least one of the second lens l 2 and third lens l 3 , each of which is made of a negative lens having an image - side surface concave toward the image , in terms of its relative partial dispersion and abbe number . the image readout lens in accordance with the present invention has favorable imaging performances within the range from about 1 / 5 × to 1 / 20 ×, in particular , for reduction , and is suitable as an image readout lens for originals on the order of the a3 - size short side , i . e ., 297 mm , japanese b4 - size short side , i . e ., 257 mm , a4 - or letter - size short side , i . e ., 216 mm , and the like .