﻿Mr. J. C. Douglas on Shadow Optometers, 341 



If A be a convex lens, B a focus of diverging rays, C an opaque 

 object interposed between B and A ; and 1, 2, and 3 a screen 



r 1 2 3 



i A 



receiving images at the different positions shown, it will be appa- 

 rent on inspection of the diagram that if the object C be moved in 

 the direction of the dotted line, the shadow will on 1 move in 

 the same direction as the object, and on 3 in an opposite direc- 

 tion, while on 2 no shadow will be thrown, but the luminous 

 point B will be reproduced diminished in intensity. If the lens 

 be replaced by the refracting media of the eye*, and 1, 2, and 3 be 

 positions of the retina, it will be evident that with the retina too 

 near A for distinct vision of B, (1) the motion of the shadow will be 

 seen in the opposite direction to that of C; and with the retina 

 too distant (3), it will appear to be in the same direction : with 

 the retina at 1 the shadow of C will appear inverted with respect 

 to C ; with the retina at 3 it will appear erect. The different 

 relative positions of the retina may be due to any cause altering 

 either the refracting media or the figure of the eye, or both, the 

 diagram being a convenient mode of representation only : e. g. 

 transfer of the retina to 1 may represent a diminution of the re- 

 fractive power of A, the retina not moving, &c. 



Applying this to optometric purposes, a method is afforded of 

 determining the effect of accommodation and refraction at any 

 moment with reference to any luminous point B. To find if the 

 retina is too near or too distant from the lens for distinct vision, 

 pass a pin or pen-point past the eye close to the cornea ; if no 

 shadow is seen, the eye is accurately adjusted ; if a shadow is 

 seen and moves in the same direction as the object, the retina is 

 too far from the lens ; if the shadow be inverted and move in the 

 opposite direction, too near; if the luminous point be far enough 

 distant for the rays to be considered parallel, the three cases are 

 emmetropia, myopia, and hypermetropia respectively. 



The degree of myopia or hypermetropia may be measured by 

 interposing lenses until the shadow is no longer seen, or lenses 

 may be applied and the luminous point approached or withdrawn 

 until the shadow is no longer seen; myopia may be measured 

 by merely approaching or withdrawing the luminous point with- 

 * A diagrammatic eye for simplicity. 



