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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



while that reflected from the anterior surface of the lens will become smaller 

 and move toward the corneal image. This change in the size and position of 

 the reflected image can only mean that the surface from which the reflection 

 takes place has become more convex and has moved forward. Coincident 

 with this change a contraction of the pupil will be observed. 



An apparatus for making observations of this sort is known as the phako- 

 scope of Helmholtz (Fig. 218). The eye in which the changes due to accom- 

 modation are to be observed is placed at an opening 

 in the back of the instrument at C, and directed al- 

 ternately to a needle placed in the opening D and 

 to a distant object lying in the same direction. Two 

 prisms at B and B' serve to throw the light of a 

 candle on to the observed eye, and the eye of an 

 observer at A sees the three reflected images, each 

 as two small square spots of light. The movement 

 and the change of size of the image reflected from 

 the anterior surface of the lens can be thus much 

 better observed than when a candle-flame is used. 

 The course of the rays of light in this experi- 

 ment is shown diagrammatically in Figure 219. 

 The observed eye is directed to the point A, while 

 the candle and the eye of the observer are placed 

 symmetrically on either side. The images of the candle reflected from the various 

 surfaces of the eye will be seen projected on the dark background of the pupil 



FIG. 218. Phakoscope of 

 Helmholtz. 



FIG. 219.--Diagram explaining the change in the position of the image reflected from the anterior surface 

 of the crystalline lens (Williams, after Donders). 



in the directions indicated by the dotted lines ending at a, 6, and c. When the 

 eye is accommodated for a near object the middle one of the three images moves 

 nearer the corneal image i. e. it changes in its direction from b to b' y showing 

 that the anterior surface of the lens has bulged forward into the position indi- 



