TESTING THE CHROMATIC ABERRATION. 167 



object a must appear, under the conditions represented in Fig. 96, 

 bordered on the left by blue, and on the right by red, because the 

 violet image v of any object-point appears upon the retina more to 

 the right, and is therefore mentally assigned to an object v' situated 

 more to the left. If, on the other hand, the left half of the pupil 

 be covered up, the arrangement of the coloured borders is reversed, 

 as shown in Fig. 97. When, in a dioptric image, the same coloured 

 borders are present in the same order as in refraction by the eye, 

 this refraction is further intensified ; while the converse of course 

 diminishes, and under certain circumstances cancels it. It follows 

 that the aberration of the eye may, in all cases where lateral dis- 

 placement of the differently coloured image-points is concerned, be 

 just as well attached to a*n under-corrected as to an over-corrected 

 objective-image, since the arrangement of the colours varies 

 according as the right or left half of the objective, or of the eye, is 

 effective. If, on the other hand, chromatic aberration is due partly 

 or exclusively to differences of plane of the differently coloured 

 images as, for instance, in the testing of optical power with the 

 meshwork it is evident that the eye is capable of lessening or 

 compensating only the contrary aberrations. This fact may be 

 worthy of some attention in practical Optics ; it is not, however, 

 decisive, because with organic objects the lateral displacements are 

 the most frequent, and do not admit of elimination, since they take 

 place in the most different directions. 



The most favourable combination for the retinal image must 

 therefore be a microscopic image free from aberration as the object, 

 and the usual vision through the central part of the pupil ; and 

 we conclude from these discussions that the best objective is that 

 in which the spherical and chromatic aberration is corrected as far 

 as possible. 



Whether the manufacture of achromatic or slightly over- 

 corrected eye-pieces would compensate for the outlay involved, we 

 will leave undecided. It is evident that, by the most favourable 

 combination of the eye-piece, the microscopic image gains only in 

 certain subordinate points, such as extent of field of view, &c. 



