44 



THE POPULAR SCIENCE MONTHLY. 



Copy the picture, of exact size, by scratching through the varnish, and 

 then blacken the lines with ink. Hold the transparent plate at the 

 distance of a foot from your eyes, and through it look at a point about 

 five feet away. Very little motion of the plate is needed to get this 

 point exactly aligned with each of the dots within the circles by look- 

 ing with each eye in succession. Look at the point now with both 

 eyes, and you will see, suspended in the air, probably just beyond the 

 plate, apparently a solid cone of glass pointing toward you, the very 

 facsimile of our glass cone from which the pictures were taken. 



Copy the picture also on paper or card-board, of exact size, but with 

 the part marked R transferred to the left, and that marked L to the 

 right. Hold up the point of a pencil about half-way between your 

 eyes and the card. In a moment the proper position is found, where it 

 is aligned with R for the right eye and with L for the left. Open both 

 eyes and converge them upon the pencil-point. A little cone pointing 

 toward you is suspended in the air just beyond the pencil, which may 

 now be withdrawn. Move your head from side to side : the cone 

 moves with you. It is brilliantly lustrous, sharp in outline, and much 

 smaller than that previously seen. Two companion circles, one on each 

 side, are left behind on the card, and are larger than the base of the 

 suspended cone, but a little smaller than the circles originally were. 

 Their appearance is due to images of R and L which fall upon retinal 

 parts that in normal vision could not be simultaneously impressed by 



Fig. 5. The First Landscape Stereograph. 



an external single body. The sensations produced by them are hence 

 not suggestive of singleness, and each is therefore referred separately 

 outward in the direction from which the rays producing them have 

 come. Such side-images are perceived also when the glass plate is 

 employed. Try the same experiment now with the picture on the 

 page ; the miniature cone leaps off the paper into the air, but this time 

 it is hollow, for its vertex is pointed to the place from which it seems 

 to have sprung. 



