SIGHT. 573 



coloured ray possessing a different index of refraction in the same 

 medium, a lens refracts these variously coloured rays to different 

 points or foci. This effect, which is called chromatic aberration, 

 is another cause of indistinctness in images formed by lenses, for 

 from this cause they appear surrounded with coloured fringes. 

 When the eye, however, is adjusted to the proper focal distance, 

 an object appears perfectly colourless. It is evident, therefore, 

 that either the eye, when thus adjusted, is by some contrivance 

 rendered achromatic, or that the aberration is so small as to be 

 insensible. Sir D. Brewster is of the latter opinion, which was 

 also held by Dr. Maskelyne. Of those who maintain that there 

 exists a correction for chromatic aberration, Euler, Coddington, 

 and Professor Powell assert that a compensation takes place be- 

 tween the refractions of the different media of the eye, each giving 

 a different explanation ; while D'Alembert supposes that the agita- 

 tion, occasioned at any one point of the retina, extends itself into 

 the adjacent points, and, each point being thus influenced by the 

 sum of the effects due to all the coloured rays at once, perfect 

 vision is the result. When the rays do not converge accurately on 

 the retina, the dispersion is sensible ; and, from this cause, objects, 

 seen either within or beyond the proper focal distance, are seen 

 with coloured borders. 



Single vision with Puoo eyes. When we look at an object, an 

 image is formed on the retina of each eye, and yet we see only a 

 single object. Two different opinions have been maintained re- 

 specting the cause of this singleness of vision with both eyes. 

 Several distinguished philosophers, and among others Dr. Reid, 

 suppose that there is a physiological connection between certain 

 points of the two retinae, and that objects are only perceived as 

 single when their images fall on corresponding parts of the re- 

 tinae : the decussation of the optic nerve is brought as an argu- 

 ment in support of this opinion. Others, including Dr. Smith and 

 Sir D. Brewster, do not admit this supposed necessary connection 

 between corresponding points of the expansion of the optic nerve, 

 but assert that objects are seen single merely because they are 

 seen by both eyes in the same place ; single vision is, according 

 to this opinion, a necessary consequence of the law of visible 

 direction. 



Insensible spot of the retina. The retina, from its being an 

 expansion of the optic nerve, has generally been regarded as the 



