EFFECTS OF FAULTY VISION IN PAINTING. 179 



greatest tension if we adapt our eye to the nearest point. It is, on 

 the contrary, in complete repose if we adapt it to the farthest point. 

 The optical state of the eye during its adaptation for the farthest 

 point, when every effort of accommodation is completely suspended, is 

 called its refraction. 



There are three different kinds of refraction: firstly, that of the 

 normal eye ; secondly, oi the short-sighted eye ; thirdly, of the over- 

 sighted eye : 



1. The normal eye, when the activity of its accommodation is 

 perfectly suspended, is adjusted for the infinite distance; that is to 

 say, it unites upon the retina parallel rays of light. 



2. The short-sighted eye has, in consequence of an extension of its 

 axis, a stronger refraction, and unites, therefore, in front of the retina 

 the rays of light which proceed from infinite distance. In order to 

 be united upon the retina itself, the rays of light must be divergent ; 

 that is to say, they must come from a nearer point. The more short- 

 sighted the eye is, the stronger must be the divergence ; such an eye, 

 in order to see distinctly distant objects, must make the rays from a 

 distant object more divergent, by aid of a concave glass. We deter- 

 mine the degree of short-sightedness by the power of the weakest 

 concave glass that enables the eye to see distinctly at a great distance. 



3. The over-sighted, or hypermeti'opic eye, on the contrary, has too 

 weak a refraction : it unites convergent rays of light upon the retina ; 

 parallel or divergent rays of light it unites behind the retina, unless 

 an effort of accommodation is made. The degree of hypermetropy, 

 or over-sightedness, is determined by the focal distance of the strongest 

 convex glass with which objects can still be distinctly seen at a great 

 distance. 



Hypermetropy has no essential influence upon painting ; it only 

 reduces the power of application, and must therefore be corrected by 

 wearing convex glasses. This can never be avoided if the hyperme- 

 tropy is so great as to diminish the distinctness of vision. Short- 

 sightedness, on the contrary, generally influences the choice of the 

 subject of the artist and also the manner of its execution. As a very 

 small handwriting is an indication of short-sightedness, so we find that 

 artists who paint small pictures, and finish the details with great 

 minuteness, and with fine touches of the brush, are mostly short- 

 sighted. 



Sometimes the shape of the eye diverges from its normal spherical 

 form, and this is called astigmatism. This has only been closely inves- 

 tigated since Airy discovered it in his own eye. Figure to yourself 

 meridians drawn on the eye as on a globe, so that one pole is placed in 

 front : then you can define astigmatism as a difference in the curvature 

 of two meridians, which may, for instance, stand perpendicularly upon 

 each other; the consequence of which is a difference in the power of 

 refraction of the eye in the direction of the two meridians. An eye may, 



