191 1.] NIPHER— AN OPTICAL PHENOMENON. 319 



pin head between the eye and the eye-lens will produce no shadow 

 on the retina. If the glass be ^focused for a nearer object, an erect 

 shadow will appear. If focused for a more distant object, the 

 shadow will appear inverted. A hole through a card and with a 

 bright background may be viewed by means of the opera glass. The 

 hole may have any diameter from 0.05 to 1.5 cm. The distance of 

 the card must be adapted to the diameter of the hole, and may vary 

 from close contact with the object lens to three or four meters, the 

 glass being focused for a more distant object. The results are as 

 indicated above. The setting sun surrounded by bright clouds may 

 be used as an object, if viewed through the foliage of trees thirty or 

 forty meters distant, the glass being focused for an object more 

 distant than the trees. The mass of foliage will be dotted with pin 

 head shadows. Each opening through the leaves acts in a manner 

 similar to the pin hole. 



In all of the cases described, the shadow upon the retina is by 

 some mental act projected outward in space. An interesting ques- 

 tion arises concerning its apparent position. LeConte says that in 

 his experiments it appears in the hole in the card. Perhaps it would 

 be proper to say that it is seen through the hole. The hole itself 

 may have a diameter of about one third that of the pin head, and 

 the pin head then appears smaller than the hole. Its apparent size 

 depends somewhat on the diameter of the hole. 



If a pin is placed back of the card and in erect position so that 

 it is visible through the hole, it may be so placed that it has the same 

 apparent size as the shadow. If the pin is at a distance of 30 cm. 

 from the eye, and the card is at a distance of 15 cm., the shadow 

 and the pin will have the same apparent size. The appearance of the 

 inverted shadow and the erect pin is as shown in Fig. i. 



(D 



Fig. I. 



This suggests an interesting device whereby the line of sight of 

 the tw^o eyes and the capacity for muscular adjustment may be exam- 



