CH. LVI.] ERRORS OF REFRACTION 787 



object will be most defined and distinct when the pupil is narrow, 

 the object at the proper distance for vision, and the light abundant ; 

 so that, while a sufficient number of rays are admitted, the narrow- 

 ness of the pupil may prevent the production of indistinctness of 

 the image by spherical aberration. 



Distinctness of vision is further secured by the pigment of the 

 outer surface of the retina, the posterior surface of the iris and the 

 ciliary processes, which absorbs most of the light which is reflected 

 within the eye, and prevents its being thrown again upon the retina 

 so as to interfere with the images there formed. 



5. Chromatic Aberration. In the passage of light through an 

 ordinary convex lens, decomposition of each ray into its elementary 

 colours commonly ensues, and a coloured margin appears around 

 the image, owing to the unequal refraction which the elementary 

 colours undergo. In optical instruments this, which is termed 

 chromatic aberration, is corrected by the use of two or more lenses, 

 differing in shape and density, the second of which continues or 

 increases the refraction of the rays produced by the first, but by 

 recombining the individual parts of each ray into its original white 

 light, corrects any chromatic aberration which may have resulted 

 from the first. It is probable that the unequal refractive power of 

 the transparent media in front of the retina may be the means by 

 which the eye is enabled to guard against the effect of chromatic 

 aberration. The human eye is achromatic, however, only so long as 

 the image is received at its focal distance upon the retina, or so 

 long as the eye is properly accommodated. If these conditions 

 are interfered with, a more or less distinct appearance of colours is 

 produced. 



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 partly * for this reason that 

 a white square on a black ground appears larger than a black square 

 of the same size on a white ground. The phenomenon is naturally 

 more marked when the white object is a little out of focus. 



Defective Accommodation Presbyopia. This condition is due to 

 the gradual loss of the power of accommodation which is an early 

 sign of advancing years. In consequence, the person is obliged in 

 reading to hold the 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. It is due chiefly to 

 the gradual increase in density of the lens, which is unable to swell 

 out and become convex when near objects are looked at, and also to 

 a weakening of the ciliary muscle, and a general loss of elasticity in 

 the parts concerned in the mechanism. 



* The phenomenon is also partly due to what is called " spatial induction." 



