ELEMENTARY EXPERIMENTAL PHYSIOLOGY 237 



surface of lens. The images at (a) and (b) are erect ; at (c) is inverted ; 

 the image at (b) appears to be the most deeply situated of the 

 three. 



EXPERIMENT II. In a darkened room let the observer bring a 

 lighted candle near the eye of the subject, rather to one side of his 

 optic axis. The observer places 

 himself so that his eye is sym- 

 metrical in position to the candle 

 on the other side of the optic axia 

 of the subject. When properly 

 adjusted there should be observed 

 reflected from the eye of the sub- 

 jecc three images the first bright 

 and erect, reflected from the cornea ; 

 a second near the centre of the 

 pupil, but much feebler than the 

 first, and apparently the most 

 deeply situated of all the images, 

 this being reflected from the an- 

 terior surface of the lens ; a third 

 image represented by a mere spot 

 of light differs from the other two 



. . FIG. 213. The phakoscopc. 



in being inverted, ir now the 



accommodation of the subject be shifted from a far to a near point, 

 the middle image will advance but grow smaller, and will approach 

 the corneal image. The other images do not alter. 



During varying accommodation it is found that this image is the 

 only one to change, thus indicating that the change is in the anterior 

 surface of the lens. 



EXPERIMENT III. The Phakoscope. This instrument is specially 

 adapted for viewing the reflected images of Experiment II. It is repre- 

 sented in Fig. 213. Fig. 214 represents diagrammatically the arrange- 

 ment and course of the rays of light. It consists of a dark box, roughly 

 triangular in shape, with the angles of the triangle bevelled off, and 

 at S and fitted with windows (Fig. 214). 



The observer's eye is at 0, the subject's at S. At two prisms are 

 arranged vertically so as to allow two illuminated squares to fall upon 

 the eye at S. The eye at S can either be focussed for the vertical 

 needle at W t or (since this lies in an opening) for distant objects beyond 

 the opening. With the alteration of the lens corresponding to the 

 change of accommodation, the images from the anterior surface of 

 the lens will vary as in Experiment II. 



