OBSERVATIONS ON SOUNDNESS. 
339 
diverging from each point of the object, and entering the 
pupil, should converge to a focus on the retina. If, as in 
myopic or short-sighted persons, the rays converge to a focus 
before they reach the retina, from the too great convexity 
either of the lens or cornea, they impinge on that sensitive 
organ in a state of divergence, and the pencils proceeding 
from contiguous points of any object are superposed upon 
and consequently confuse each other; and the further the 
focus of incident rays is from the refracting surface of the 
eye, the further the focus of refracted rays will be from the 
retina, and consequently the greater confusion ; hence, with 
persons thus affected, the difficulty of discerning objects 
increases with their distance from the eye. The very term 
^ myopic^ is derived from the effort naturally made to 
diminish the aperture of the transmitted pencils, and conse¬ 
quently the confusion, by partly closing the eyelids, on the 
same principle that an object placed near to the eye may be 
seen distinctly through a pin-hole in a card. A concave lens, 
of suitable power, by increasing the divergence of the incident 
pencil, will diminish the convergence of the refracted rays, 
and consequently carry back each focal point towards the 
retina, and this is the kind of spectacles worn by short¬ 
sighted persons. 
“ In presbyopic or long-sighted persons, on the contrary, 
the lens or the cornea is not sufficiently convex, and as the 
foci of refracted pencils are consequently situate behind the 
retina, a similar superposition and confusion of contiguous 
pencils ensue. In this case it is necessary to diminish the 
divergence of the incident rays, which is effected by a convex 
lens; and the convergence of the refracted pencils being thus 
increased, their foci will be brought forward to the surface 
of the retina. As the term ^ presbyopic^ implies, this is the 
state of vision incidental to old age, and arises from the 
diminution in the convexity of the crystalline lens that 
naturally takes place in advancing years. Individuals 
are not unfrequently met with whose eyes are as insensible 
to certain tints as the ears of others are to particular sounds. 
Several cases of this kind have been described, in which the 
following colours have been confounded by the persons 
affected with this curious defect of the visual organs: 
Bright green, wdth grayish-bro^Yn and flesh-red. Rose- 
red, Yvith green and gray. Scarlet, with dark green and 
hair-browm. Sky-blue, with grayish-blue and lilac-gray. 
Brownish-yellow, 'with yellowish-bro'wn and grass-green. 
Brick-red and rust-bro'v\m, wdth deep olive-green. Dark 
violet, with deep blue. This remarkable state occasionally 
