EXPERIMENTS ON THE DIOPTRIC MECHANISM 



103 





n 



G - si. PHAKOSCOPE ; a, SITUATION- OP 

 OBSERVED BYE ; 6, DITTO OP OBSERVER'S 

 ETE ; c > LENSES ; n, APERTURE WITH 

 NEEDLE. 



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. 81) 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 posterior surfaces 

 of the lens respectively, can, of 

 course, only be seen within the 

 pupil. The last pair is difficult to 

 make oat. If the subject be asked, first to look through the needle n 

 at a distant window and then at the needle, the middle pair of images 

 become smaller and slightly brighter ; they also approach each other 

 and come nearer to the corneal images during accommodation for 

 the near object ; the other two pairs remain unaffected. 



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

 twelve inches long is marked in inches or centimeters. One end of 

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

 inches off. If the eye is normal, it 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 five inches without its image being blurred ; 

 when this occurs the near point for that eye has been passed. 



If the eye is hypermetropic the needle will already appear blurred 

 at five inches, and it may be required to be moved considerably 

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

 passed. 



Pater Scheiner's experiment. The observation is rendered easier 

 and more striking by Scheiner's device of observing the needle 

 through two pinholes made close to one another and side by side in 

 a card fixed vertically at one end of the scale (fig. 82). In this 



