162 EXPERIMENTAL PHYSIOLOGY. 



reflected on a wall and the mirror reflecting it is rotated upward, the movement of 

 the spot is in the same direction as that of the mirror. If a spot of light be 

 reflected into a normal eye and the mirror moved, the movement of the light on 

 the retina appears to the observer's eye, placed at the hole in the instrument, also 

 to be in the same direction. This is shown in Fig. 45, No. 1 A is the first 

 position of the spot of light and B the second, moved by an upward movement of 

 the mirror. The rays from A, when after refraction they come out of the eye, 

 appear to come from A' at infinity. Those arising from B seem to come from B', 

 also at infinite distance. The mirror was moved up and the image also moves up. 

 If a strong converging lens is held in front of the observed eye so that the rays 

 coming out are made to form inverted images, at some short distance from the 

 lens, of the retinal points from which they arise, and if the rays are diverging from 

 the images again when they reach the observer's eye, the condition is reversed. 

 The spot of light appears to move in a direction opposite to that of the movement 

 of the mirror. This is shown in Fig. 45, No. 2. The rays from A, converged after 

 coming out of the eye, make the image at A', those from B at B'. If the observer's 

 eye be farther away than A' and B', the rays from the first (lower) position of the 

 spot of light seem to come from the upper image, those from the second place of 

 the spot, which was really above the first, from an image lower than the first image. 

 The direction of movement is thus reversed. If the observer's eye be nearer than 

 the focus of the lens (that is, nearer than A' or B'), the movement appears to be 

 in the direction of the movement of the mirror, as it was without the lens. Sup- 

 pose the observer's eye is always at a distance of one metre from the subject. 

 Then if the lens is stronger than one dioptre (i.e., has a focal length of less than 

 one metre) the spot will move in the opposite direction. If the lens is weaker than 

 this there will be no reversal, and the spot will move in the same direction as the 

 mirror. Now, if the subject's eye is not normal but long-sighted, the rays when 

 they come out diverge from each other. To bring these to a focus in front of his 

 eye, still at one metre distance, the observer will have to use a stronger lens than 

 before and the more long-sighted the eye is, the stronger the lens will have to be. 

 Rays coming from a short-sighted eye are already converging when they come out. 

 If the defect is small, a converging lens will still be needed to make the movement 

 of the spot appear reversed to an observer at one metre, though not so strong a one 

 as is needed for the normal eye. If the defect is great, the focus of the rays 

 without any lens at all may lie at some point between observer and subject, and 

 in this case the spot of light moves in the opposite direction to the mirror from 

 the first. The condition will be thus like that already described for Fig. 45, 

 No. 2, only that no lens need be imagined. A divergent lens must be used to 

 cause it to go to the similar direction and, as before, the strength of the lens which 

 must be used gives a measure of the abnormality. An examination of this kind 

 can be done in any plane of the eye and the method can therefore be used to 

 measure astigmatism. In this case the strength of lens necessary to give reversal 

 when one plane of movement is being tested is different from that for another 

 plane. 



