DIOPTRICS OF THE EYE. 329 



this retina is in turn sent back toward the mirror, its path being indicated for 

 the point b by the black Unes. If the eye is emmetropic the rays from this 

 point emerge parallel, and, faUing upon the biconvex lens, are brought to a 

 focus at b'. Similarly the rays from a will be brought to a focus at a' and 

 from c at c'. Consequently there will be formed in the air an inverted image, 

 and it is at this image that the eye of the observer gazes through the hole in 

 the mirror. This image forms its image on the retina of the observer's eye, 

 as represented in the diagram at a", b" , c", and is projected outward or seen 

 inverted as regards the original position of the points in the retina of eye /. 

 The indirect method gives a larger field than the direct method, although the 

 objects seen are of smaller size. 



The Retinoscope or Skiascope. — When one reflects a spot of light 

 upon a wall, any movement of the reflecting (plane) mirror is followed by a 

 movement of the reflected spot in the same direction. So if the fundus of the 

 eye is illuminated by a plane mirror provided with a peep-hole, the observer 

 looking through this hole may see a spot of light reflected from the retina 

 and can determine whether the spot moves in the same direction as the 

 mirror or against it. If the eye under observation is normal (emmetropic), 

 then the rays of light starting from the retina will emerge in parallel bundles, 

 since the retina lies at the principal focal distance, and as the mirror is tilted 

 from side to side the illuminated spot moves in tlie same direction. By placing 

 a convex lens of suitable focus in front of the observed eye we can cause the 

 emerging parallel rays to come to a focus and cross before reaching the ob- 

 server's eye. In such a case the movements of the spot of light upon the retina 

 will be against those of the mirror. For example, let us suppose that the 

 observing eye is placed just 1 meter away from the eye observed, then if 

 we put in front of the latter a convex lens of 1.25 D. the emerging rays will be 

 focused at a point 25 ctm. in front of the observer's eye and the movements 

 of the spot of light will be against the mirror. A lens of less than 1 D. placed 

 in front of the observed eye would not bring the rays to a focus in front of 

 the observer's retina, consequently the movements of the spot would be with 

 the mirror. Assuming that we are dealing with an emmetropic eye, it can 

 be shown that at the distance mentioned (I meter) any lens of less than 

 1 D. placed in front of the eye leaves the movements with the mirror, 

 while any lens of more than 1 D. gives movements against the mirror. 

 Consequently a lens of just 1 D. would mark the exact "point of rever- 

 sal." With a lens of this power the focus would fall theoretically just on the 

 observer's retina. In such a case any movement of the mirror would be 

 followed by the appearance or disappearance of the spot, but no direction of 

 movement would be perceived. The movements of the spot of light formed 

 upon the retina by the retinoscopic mirror may be u.sed to determine all the 

 various abnormalities of refraction of the eye according to the following 

 general schema : The observer sits at a fixed distance, say 1 meter, from 

 the patient, and determines whether the reflected spot from the illuminated 

 fundus moves with or against the mirror. If the movement is with the mirror, 

 then the eye under observation is either normal or hyperopic (or if myopic 

 the myopia is less than 1 D.). By placing convex lenses in front of the eye 

 the observer seeks for the point of reversal. If this point is given by a lens 

 of + 1 D., then the eye under examination is emmetropic ; if a stronger lens 

 is required the eye is hyperopic, that is, the emerging rays are divergent and 

 require a stronger lens to bring them to a focus before reaching the observer's 

 eye. In the latter case the amount of hyperopia is obtained by ascertaining 

 the strength in diopters of the lens required to just reverse the movement 

 and subtracting 1 D. from it, since the latter amount is required, at a distance 

 of 1 meter, to get reversal with the normal eye. If the reversal is given by 

 a convex lens of less than ID., then the eye is myopic to an extent less than 

 1 D. Wlien the movements of the spot of light are against the mirror 

 from the beginning, then the observer is dealing with a myopic eye (the myopia 

 being greater than ID.). To reverse the movement it is now necessary to 

 place concave lenses in front of the observed eye until the point of reversal 

 is obtained, that is, until the focus of the emerging rays falls behind the 



