THEOEY OF MICROSCOPICAL VISION. 19 



want of perfect achromatism is a fact somewhat analo- 

 gous to that belonging to the flint and crown-glass 

 construction of the lenses of optical instruments. On 

 the other hand, with regard to its spherical aberra- 

 tion straying away of the rays of light it is said 

 that the great mobility of the iris corrects this 

 defect. The iris acts somewhat as the diaphragm 

 does in the microscope, shuts off the circumferen- 

 tial rays of light those rays which, straying away, 

 produce distortion of images in lenses, and increase 

 the circles of dispersion over the retina. Luminous 

 rays on entering the eye are partly absorbed and partly 

 reflected, and on issuing once more follow the same 

 course as they did in the first instance. A certain por- 

 tion, however, of each bundle of axial rays, after 

 having undergone refraction, are brought to an accu- 

 rate focus on points of the retina, and excite a limited 

 number of the outer layer of rods and cones. The 

 sharpness with which the aerial image is seen depends 

 upon the magnitude of the retinal image and the diame- 

 ter of the visual angle. Other considerations enter into 

 the theory of vision, as that of the situation of the 

 retinal image, etc. By education and experience we 

 "become acquainted with the fact that objects are not 

 so well defined under a small visual angle, and for 

 seeing minute objects it is absolutely necessary to 

 resort to artificial means. Withal, to view the infi- 

 nitely little in all their beauty of form and complexity 

 of design, we require, for the most part, associated 

 with light, a difference and intensity of brightness and 

 of colour; for the delicacy of visual perception is 

 found to depend less upon the number of the retinal 

 elements, set in motion by the waves of light, than 

 upon the number of elements capable of appreciating 

 and separating the many delicately coloured tints, 

 which are embraced by the images. (Hermann.) 



Theory of Microscopical Vision. 



Recently, however, Professor Abbe, of Jena, a high 

 authority on all that relates to microscopical optics, 

 has propounded a theory of the formation of the images 

 C 2 



