722 HANDBOOK OF PHYSIOLOGY. 



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

 Helmholtz 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 adjusted 

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



From the insufficient adjustment of the image of a small white ob- 

 ject, it appears surrounded by a sort of halo or fringe. This phenom- 

 enon 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 

 a white ground. 



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

 following, 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 a flat 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). 



Defective Accommodation Presbyopia. This condition is due to the 

 gradual loss of the power of accommodation which is part of the general 

 decay of old age. In consequence the patient would be obliged in read- 

 ing to hold his book further and further away in order to focus the 

 letters, till at last the letters are held too far for distinct vision. The 

 defect is remedied by weak convex glasses, which are very commonly 

 worn by old people. It is due chiefly to the gradual increase in density 

 of the lens, which is unable to swell out and become convex when near 



