348 ABERRATIONS OP LIGHT. 



of very indistinct images, were it not for the diaphragm 

 and the black lining of this optical instrument. These 

 imperfect images are termed aberrations of light, and are 

 reduced to three, i. e., the aberration from sphericity, from 

 parallax, and chromatic aberration. Now nature has pro- 

 vided in the living eye contrivances much more perfect, in 

 obviating these several varieties, than any to be found in 

 the workmanship of man. These consist of the iris, the 

 ciliary processes, the choroid membrane, and the lenses of 

 different refracting power. The aberration of sphericity is 

 corrected by the iris, whose black pigment intercepts all 

 the lateral rays which fall upon the cornea, and allows 

 only those to enter which are in the axis of vision. Were 

 it not for this provision, all the rays falling on any por- 

 tion of the cornea, not in a line with the pupil, would 

 converge sooner and form different focal points, and neces- 

 sarily produce indistinct vision. 



Aberration from parallax, as it is termed, is caused by 

 the rays of light coming from distant objects in parallel 

 lines, and being consequently brought to a focus soon, and 

 near the lens, while the rays from near objects are diver- 

 gent and do not come to a focus for a much greater dis- 

 tance; for the law is, the further the object the nearer the 

 focus to the lens; the focal distance behind the lens corre- 

 sponds to the distance of any object. The eye, it is known, 

 has the power of accommodating itself to, and seeing at, 

 different distances but in what this power consists (and 

 where it resides) is not exactly agreed. Some suppose the 

 lens to be muscular, and thereby capable of altering its 

 form, density and distance from the retina, and thus al- 

 ways producing the proper focus at any distance at all 

 within the range of vision. By others the muscles, acting 

 upon and compressing the globe of the eye, and thus 

 changing its form, are supposed to furnish this power. 

 Others again assign the cause to a change in the con- 

 vexity of the cornea. 



Chromatic aberration, or the confusion in sight arising 

 from the decomposition of the light in its passing through 



