LONG AND SHORT SIGHT. 445 



On now looking at a perpendicular line, through these holes, the line 

 appears double if the eyes be too close to it, but single at the distance 

 of perfect or normal vision, which distance is thus ascertained for any 

 particular eyes. It is desirable in the selection of glasses, not to 

 over-correct the natural defect, for this would fatigue and weaken the 

 eyes still more. The eyeglass, or spectacles, should merely render 

 objects distinct, but not magnify or diminish them. Concave glasses 

 are numbered according to the distance of their virtual focus from 

 them. By multiplying the normal distance of near vision, say 10 

 inches, by the distance of clear vision in the short-sighted person, say 

 4 inches, and dividing the product by the difference between those two 

 factors, the number of the concave glass required is found ; thus, 

 10x4-7-6=6.6; i. e., about No. 7 glass. The same rule obtains in 

 the choice of convex glasses for long-sighted people. In the normal 

 eye, concave glasses diminish the size of objects looked at through 

 them, because they diminish the actual size of the retinal image. On 

 the other hand, convex glasses, or lenses, including both simple and 

 compound microscopes, increase the apparent size of objects, by en- 

 larging their images on the retina ; they, in fact, enable the eye to 

 see such objects under large angles, or as if they were very closely 

 approximated to the eye. The necessary dilution of the light, in this 

 process, is met by various contrivances for powerful artificial illumi- 

 nation. 



A distinction has been drawn between short and long sight, or so- 

 called myopia and presbyopia, on the one hand, and near and far 

 sight, or true myopia and presbyopia, on the other. The former 

 states depend, as we have seen, on individual peculiarities in the shape 

 of the eyeball or its parts ; for, besides normally-constructed eyes, 

 there are eyes, the natural foci of which lie either in front of the 

 retina, helping the eye in near vision, or behind the retina, adapting 

 it for distant vision. The causes may be a greater or less prominence 

 of the cornea, a shorter or longer optic axis, and a greater or less 

 curvature of the lens. Near sight and far sight depend on defects in 

 the power of accommodation of the eye ; as, for example, on loss of 

 power in the ciliary muscle and iris, or on diminution of the elasticity 

 of the lens or its suspensory ligament, both of which conditions are 

 found in advancing age. In short sight, the point of nearest vision 

 may be even as close as two inches to the eye ; but in near sight, or 

 true myopia, the near point may be 12, 30, or even more inches from 

 the eye. The term hypermetropia has been used to designate the con- 

 dition of long sight, sometimes named presbyopia, which latter term 

 is then restricted to the impairment of vision, as regards near objects, 

 which comes on after a certain period of life, and which is due to a 

 diminution in the range of the power of accommodation (Bonders.) 



In many individuals, the focal lengths of the two eyes are different, 

 a fact which escapes attention, unless the difference be very marked. 

 It is worthy of observation that short-sighted eyes are still achromatic, 

 objects seen indistinctly by them being without colored fringes. Ac- 

 cording to Ehrenberg, the absolute limits of vision are in no way de- 



