EXPERIMENTS ON THE DIOPTRIC MECHANISM 



119 



image reflected from the front of the lens becomes smaller and moves 

 nearer to that reflected from the front of the cornea; the other 

 images remain unaltered. This change of the second image is due to 

 bulging of the anterior surface of the lens. 



3. Phakoscope. The same experiment may be performed with a 

 phakoscope (Fig. 88) with less trouble, since all the points are fixed. The 

 instrument a triangular box with truncated angles a, b, c is used 

 in a darkened room. A lamp is 

 placed in such a position that the 

 light from the two square window 

 prisms at c falls upon the observed 

 eye at a. The observer looks through 

 the opening at b and sees in the 

 observed eye three pairs of images 

 two bright squares (reflected from 

 the anterior surface of the cornea), 

 two larger but less distinct squares, 

 and two smaller and much dimmer 

 squares. The two last pairs, being 

 reflected from the anterior and pos- 

 terior surfaces of the lens respec- 

 tively, can, of course, only be seen 

 within the pupil. The last pair is 

 difficult to make out. If the subject 



be asked, first to look past the needle n at a distant window, and then 

 at the needle, the middle double image becomes smaller and slightly 

 brighter during accommodation for the near object; the squares 

 approaching each other and coming nearer to the corneal image ; the 

 other two double images remain unaffected. 



Near and far points of distinct vision. A wooden scale about 

 12 inches long is marked in inches or centimetres. One end of 

 this is placed close to the eye, and a needle is put in about 5 inches 

 off. If the eye is normal, the needle should be seen sharply at 

 this distance and at any point beyond; but if it is brought nearer 

 the eye its image becomes blurred. If the eye is myopic the needle 

 may be brought nearer than 5 inches without its image being 

 blurred ; when blurring occurs the near point for that eye has been 

 passed. 



If the eye is hypermetropic the needle will already appear 

 blurred at 5 inches, and it may be required to be moved considerably 

 further from the eye before the near point of distinct vision is 



Pater Scheiner's experiment. The observation is rendered easier 



FIG. 88. Phakoscope : a, Situation 

 of observed eye ; b, ditto of ob- 

 server's eye ; c, lenses ; n, aperture 

 with needle. 



