320 NIPHER— AN OPTICAL PHENOMENON. [Api-ii 21, 



ined. Pierce a card with two pin holes, at such a distance from each 

 other that when placed at half the distance of distinct vision from 

 the eyes, they may be seen as one. This can be done by drawing lines 

 across the ruled lines of a page of white paper, and crossing the 

 ruled lines symmetrically so that at the top of the page the lines are 

 farther apart and at the bottom they are nearer together than the 

 two eyes. Pierce pin holes at each intersection of the ruled lines 

 with the cross lines. If held in front of the eyes so that the cross 

 lines are seen double, the two inner images of the lines will appear 

 to cross. At this distance apart thus determined two holes will 

 appear as one. Place a card having holes thus placed in front of the 

 eyes. Mount two pins in front of the pupils so that the two shadows 

 appear superposed in the superposed images of the holes. Two pins 

 may now be placed back of the card so that when viewed through the 

 holes they will also appear superposed. The two holes and the four 

 pins will then present the appearance shown in Fig. i. This ar- 

 rangement locates two points along the line of sight of each eye. 

 The holes may be in separate cards which close the ends of two 

 tubes, through which the observations are made. These tubes, 

 together with the pins, should be capable of screw adjustments. 



When the pin hole is viewed through a tube which is lined with 

 dark paper, the card serving to close the outer end of the tube, it may 

 be used for an examination of certain imperfections in the eye. For 

 exauTple, in my own case one eye shows a minute hole with a bright 

 background to be of uniform appearance. \'iewed by the other eye 

 a rather sharply defined shadow is shown in the center of the hole. 

 This is due to a slight irregularity in the curvature of the outer sur- 

 face of the cornea. This is due to a grain of gunpowder which was 

 blown into the eye from a horse-pistol which was discharged from a 

 distance of about 35 cm., into the lower part of the face, about 

 fifty years ago. The grain of powder w^as visible for many years, 

 but has been gradually absorbed. A slight distortion of closely ruled 

 parallel lines indicates that an irregularity of the surface still per- 

 sists. The shadow seen in the pin hole shows that light is not uni- 

 formly spread over the retina when a slightly divergent beam of 

 light enters the pupil. Any opacity in the crystalline lens would 

 also produce a shadow upon the retina. 



