OF THE SENSE OF SIGHT. 123 



meet the surface of the lens enclosed in its capsule, and are 

 thus brought to a focus, which falls on the retina, the rays 

 having in the mean time passed through the vitrine or 

 vitreous humour of the eye. 



234. It has been proved by many simple experiments 

 that the image is represented on the retina, and that it is 

 reversed ; the following figure (Fig. 80) explains how this 

 happens. The rays of light proceeding from the point a fall 

 on the retina at b ; those from c impinge the retina at d ; 

 thus the object is precisely reversed as painted on the retina. 



Fig. 80. 



235. The pigment covering the posterior surface of the 

 iris, and all the inner surface of the choroid, is necessary for 

 perfect vision, as may be seen by examining the eyes of 

 albinos : it is only at night that their vision becomes 

 distinct. 



236. The globe of the eye is the most perfect of all 

 optical instruments ; for whilst it is in general achromatic, 

 it presents no aberration of sphericity,* and its range is con- 

 siderable. It possesses the power of adaptation to various 

 distances in its normal condition, although it has not been 

 satisfactorily explained on what structure this depends. 

 Within a short range it is obviously connected with the 

 presence of two eyes. 



In some the eyes have not this power. Presbyopia de- 

 pends apparently on a deficiency of convergence of the rays of 

 light whilst passing through the humours of the eye ; hence 

 persons so affected do not see distinctly objects near at hand. 



* By achromatism is meant the power of causing the light to deviate from 

 its course without developing or decomposing its rays into their primitive 

 colours. 



The aberration of the sphericity consists in the reunion of the rays of light 

 which fall on different parts of a lens with foci distinctly different, whence 



perform this function of correction. 



