SPHERICAL ABERRATION. 649 



correction is effected, in both cases, by the iris, which 

 forms a kind of annular diaphgram to cover the circum- 

 ference of the lens, and to prevent the rays from passing 

 through any part of the lens but its centre, which corre- 

 sponds to the pupil. 



The image of an 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 suffi- 

 cient number of rays are admitted, the narrowness of the 

 pupil may prevent the production of indistinctness of the 

 image by this spherical aberration or unequal refraction just 

 mentioned. But even the image formed by the rays pass- 

 ing through the circumference of the lens, when the pupil 

 is much dilated, as in the dark, or in a feeble light, may, 

 under certain circumstances, be well defined; the image 

 formed by the central rays being then indistinct or invisible, 

 in consequence of the retina not receiving these rays where 

 they are concentrated to a focus. 



Distinctness of vision, is further secured by the inner 

 surface of the choroid, immediately external to the retina 

 itself, as well as the posterior surface of the iris and the 

 ciliary processes, being coated with black pigment, which 

 absorbs any rays of light that may be reflected within the 

 eye, and prevents their being thrown again upon the retina 

 so as to interfere with the images there formed. The 

 pigment of the choroid is especially important in this 

 respect ; for the retina is very transparent, and if the sur- 

 face behind it were not of a dark colour, but capable of 

 reflecting the light, the luminous rays which had already 

 acted on the retina would be reflected again through it, 

 and would fall upon other parts of the same membrane, 

 producing both dazzling from excessive light, and indis- 

 tinctness of the images. 



In the passage of light through an ordinary convex lens, 

 decomposition of each ray into its elementary coloured 

 parts commonly ensues, and a coloured margin appears 



