VISION AND THE TECHNIQUE OF ART. 21 



being corrected for oblique astigmatism means that the image lines aa 

 and bb, Figure 22, are brought together to a point on the image plane. 

 There is therefore none of the characteristic stretching in tangential 

 and radial directions which was so evident in the oblique images 

 formed by the other lens. The lens also being corrected for chromatic 

 aberration there is no substantial difference in the form of the images 

 for the different colors. They are all imaged as small spots of light 

 of about the same size. The marked difference in imaging from that 

 which occurs in the eye as shown in Figure 24 (b) should be noted. 



Figure 24 (d) and (f) are photographs made with a corrected lens of 

 white light point sources at B and D, Figure 23. Due to the correc- 

 tions the images do not show any stretching in a radial or tangential 

 direction and they are all the same shape and size for different colors. 

 The combined images will therefore appear simply as diffused white 

 spots. The marked difference between this imaging and that shown 

 in Figure 24 (a) and (c) should be noted. 



It should also be noted here, that while in the imaging of point 

 sources by the lens having substantially the same aberrations as the 

 eye the images have characteristic forms and colored fringes due to 

 their distances from the lens, in the imaging by the corrected lens, 

 although there is a diffusion which may indicate that the point source 

 is not in the object plane, there is nothing to indicate which side of the 

 object plane it is or how far it is from it. 



In order to show the characteristic chromatic edges produced in the 

 images of black and white objects, colored photographs of a black cross 

 on a white background and of a white cross on a black background were 

 taken with a lens having approximately the same chromatic aberration 

 and oblique astigmatism as the eye, and also with a corrected lens. 

 They are shown in Figures 25, 26, 27, 28, and 29. The photographs 

 taken with the corrected lens will be considered first as they show the 

 exact shape of the black and white crosses. 



Figure 28 was taken with a corrected lens of the crosses placed at C, 

 Figure 23. The blue, yellow and red images are seen to be all of the 

 same size and shape, the combined image will therefore be white and 

 black with no chromatic edges. 



Figure 25 was taken with a lens having approximately the same 

 chromatic aberration and astigmatism as the eye, of the crosses at C 

 as in Figure 28. Being beyond both the primary and secondary 

 astigmatic fields for blue the blue is generally diffused. Being in the 

 secondary field for yellow the yellow horizontal or radial lines, both 

 black and white, are relatively sharp, the vertical or tangential ones, 



