318 NIPHER— AN OPTICAL PHENOMENON. [April 21, 



cular images like pin hole images will be seen between the crossed 

 fibers. Some of these are due to the right eye and some to the left. 

 A pin head in front of either eye will show multitudes of inverted 

 pin head shadows. 



A circular disk of white paper having a diameter of i mm. or 

 slightly less, mounted upon a black card will also have upon it a 

 sharply defined black shadow of the pin head, if the side facing the 

 observer is illuminated. The paper disk must be near enough to 

 the eve so that its image on the retina is out of focus, as in other 

 cases where the pin hole is used. At various points on the glowing 

 end of a cigar, when observation is made in a darkened room, similar 

 shadows may be observed. A small blot of ink on a sheet of white 

 paper will yield a white shadow of the pin head. The same result 

 is given bv a hole in a white card, if the card is illuminated and 

 observation is made through the hole at a dark background. 



If the reflected image of the full moon or of a bright star from 

 the convex surface of a lens be used instead of the pin hole in a card, 

 the inverted shadow will be observed. If the reflecting surface is 

 concave, the shadow will appear erect if the eye is placed between 

 the reflector and its principal focus. If the eye and pin are in the 

 divergent beam beyond the principal focus, the shadow of the pin 

 head will appear inverted. 



It is evident that when the shadow on the retina is erect, it appears 

 inverted, and I'icc versa. 



The eye lens and retina may be replaced by a convex lens and a 

 paper screen upon which an image of the moon may be cast. A pin 

 closelv in front of the lens will show no .shadow. If another convex 

 lens be now placed in front of the lens representing the eye, the 

 moon's image will be out of focus. The moon's image may be in 

 front of or behind the screen, according to the position of the second 

 lens. The shadow of the pin will then appear. 



The capacity for accommodation of this artificial eye is unlim- 

 ited, and the second lens may be dispensed with. The screen being 

 placed between the lens and the image, the shadow of the pin will 

 appear erect on the screen. When placed beyond the image, it will 

 appear inverted. 



If an opera glass be focused on a street lamp 50 meters away a 



