VI 



DIOPTKIC MECHANISM OF THE EYE 



301 



More exact measurements can be made with the optometers of 

 Porterfield or Stampfer, which are constructed on the principle of 

 Scheiner's experiment. This consists in looking at a pin at 

 different distances from the eye, through two small holes in a card 

 or metal plate, made so close together that they both fall within 

 the diameter of the pupil. The pin appears single, with sharp 

 outlines, even if only weakly illuminated, when it is at such a 

 distance that its image falls upon the outer layer of the retina. 

 When the distance is greater or less it no longer appears as one 

 pin but as two, with rather blurred outlines due to diffusion circles, 

 although still fairly distinct because they arise from narrow, 

 luminous cones. 



This physiological experiment can be diagrammatically repro- 

 duced with a biconvex lens, a screen with two holes, and a 

 receiving surface on which the flame of a candle is projected. In 

 Fig. 136, ef represent the two holes in the screen that corresponds 



Fie. 136. Diagram to illustrate Scheiner's experiment for determining the near point and far 

 point of distinct vision, i.e. the range of accommodation. 



to the pupil, rr the retinal plane on which the image / of the 

 object is projected. The image is single when the plane rr 

 coincides with the seat of the image ; when, on the contrary, it 

 lies more in front (r'r'} or behind (rV) two images are formed, 

 owing to diffusion circles e'f e"f". On closing hole e or / one or 

 the other image disappears, either on the same or the opposite side, 

 according as the double image is formed before or after the 



O C5 



crossing of the rays. In experiments with the eye, owing to the 

 image being inverted on the retina, the position of the images is 

 the reverse of that in the figure, i.e. e is below and e" above, or /' 

 above and/'" below. 



Starnpfer's optometer, constructed on the principle of 

 Scheiner's experiment, serves principally for the determination 

 of the far point. It consists of two tubes fitting one over the 

 other like those of a telescope. One of the tubes which is 

 brought close to the eye carries a convex lens of moderate focal 

 length (8 D.), which makes the eye to which it is applied highly 

 myopic, so as to place the mechanism of accommodation in 

 complete rest. In front of this lens there is an opaque diaphragm 



