178 UNIVERSITY OF COLORADO STUDIES 



lined can very easily be made by anyone, as a stereoscope and a few 

 mirrors are always available. 



Figs, i, 2, 3, and 4 show how a stereograph can be made. 1 If you 

 fasten your head in a head-rest and place a lamp-shade with the narrow 

 end toward you, as is represented in Fig. 1, there will be two cones of 

 light entering each of the two eyes (X and Z) looking at the globe. 

 Every point of the shade (e. g., A) will send a ray of light into each of 

 the two eyes. So all the rays of light entering each eye from the large 

 circle AB form cones with the apices at Z and X. The same is true 

 of all the rays passing to the eyes from the circumference of the little 

 end. These cones will be inside the larger cones represented by the 

 rays from circumference AB. Now if a plane be passed in front of the 

 eyes so as to intercept the cone- forming rays, those rays will form upon 

 the plane for each eye two circles. In Fig. 3, where the two circles 

 looked at, i. e., AB and CD, are in the same plane and concentric, the 

 two cones for each eye are concentric and the two circles for each eye 

 on the interposed plane are concentric. In Fig. 1, where the little end 

 CD of the lamp-shade looked at, lies in a plane nearer the eyes than the 

 big end AB, and in Fig. 2, where the little end CD lies in a plane 

 farther away than the plane of the big end ; in neither of these two cases 

 are the two cones of rays for each eye, nor the circles formed on the 

 interposed plane, concentric. In Fig. 1, the little circle cd, for the right eye, 

 is eccentric in the direction of a of the circle ab, and in the left eye in the 

 direction of b of the circle ab. In Fig. 2 the little circles cd for each 

 eye are eccentric in just the opposite directions from those in Fig. 1, 

 for the reason that in one of the figures the little end of the shade looked 

 at is toward you and in the other away. 



In Fig. 4, the following experiment can be performed to show how 

 the eccentricity changes with reference to the little end CD of the object 

 looked at. A ring AB can be taken, across which a rubber dam can be 

 stretched and stitched along AB. Concentric with AB can be sewed 

 on the dam a smaller ring CD. By placing this object in the proper 

 position for this experiment, and letting the dam assume the form of 



1 By stereograph is meant the card, with two images upon it, which, looked at through the stereoscope 

 gives the effect of solidity. 



