214 HAND-BOOK OF PHYSIOLOGY. 



conditions be interfered with, a more or less distinct appearance of colors 

 is produced. 



An ordinary ray of white light in passing through a prism, is refracted, 

 i.e., bent out of its course, but the different colored rays which go to 

 make up white light are refracted in different degrees, and therefore 

 appear as colored bands fading off into each other: thus a colored band 

 known as the "spectrum" is produced, the colors of which are arranged 

 as follows: red, orange, yellow, green, blue, indigo, violet; of these the 

 red ray is the least and the violet the most refracted. Hence, as Helm- 

 holtz has shown, a small white object cannot be accurately focussed on 

 the retina, for if we focus for the red rays, the violet are out of focus, 

 and vice versa : such objects, if not exactly focussed, are often seen sur- 

 rounded by a pale yellowish or bluish fringe. 



For similar reasons a red surface looks nearer than a blue one at an 

 equal distance, because, the red rays being less refrangible, a stronger 

 effort of accommodation is necessary to focus them, and the eye is ad- 

 justed as if for a nearer object, and therefore the red surface appears 

 nearer. 



From the insufficient adjustment of the image of a small white object, 

 it appears surrounded by a sort of halo or fringe. This phenomenon is 

 termed Irradiation. It is from this reason that a white square on a 

 black ground appears larger than a black square of the same size on white 

 ground. 



As an optical instrument, the eye 'is superior to the camera in the fol- 

 lowing, among many other particulars, which may be enumerated in 

 detail. 1. The correctness of images even in a large field of view. 2. 

 The simplicity and efficiency of the means by which chromatic aberra- 

 tion is avoided. 3. The perfect efficiency of its adaptation to different 

 distances. In the photographic camera, it is well known that only a com- 

 paratively small object can be accurately focussed. In the photograph 

 of a large object near at hand, the upper and lower limits are always more 

 or less hazy, and vertical lines appear curved. This is due to the fact 

 that the image produced by a convex lens is really slightly curved and can 

 only be received without distortion on a slightly curved concave screen, 

 hence the distortion on aflat surface of ground glass. It is different with 

 the eye, since it possesses a concave background, upon which the field of 

 vision is depicted, and with which the curved form of the image coincides 

 exactly. Thus, the defect of the camera obscura is entirely avoided; 

 for the eye is able to embrace a large field of vision, the margins of which 

 are depicted distinctly and without distortion. If the retina had a plane 

 surface like the ground glass plate in a camera, it must necessarily be much 

 larger than is really the case if we were to see as much; moreover, the 

 central portion of the field of vision alone would give a good clear picture. 

 (Bernstein.) 



B. Defective Accommodation Presbyopia. This condition is 

 due to the gradual loss of the power of accommodation which is part of 



