592 HANDBOOK OF PHOTOGRAPHY 



Improper mounting may cause distortions: 



If both views are rotated around their individual centers in the same direction 

 the entire scene is twisted in that direction. This is liable to occur if mounting is done 

 by eye, for, if the first print is not level, the second will be rotated to obtain 

 fusion. 



If the views are rotated in opposite directions, say the bottoms separated and the 

 tops brought together, then the bottom area will recede and become smaller while the 

 top area approaches and becomes larger. 



If the stereograph is improperly placed in the stereoscope, the following distortions 

 may occur: If it is tipped forward or backward from the vertical, the image loses height 

 while the approaching portion comes nearer. 



If the stereograph is not parallel to the line between the lens centers while fusion is 

 maintained, the image is twisted in the same direction while losing width. 



An occasional individual has no capacity for stereoscopic vision. The chief rea- 

 sons for this are the habit of depending on a master eye or the fact that one eye is 

 considerably weaker than the other. No accurate figures are available, although the 

 number of such individuals is said to be in the neighborhood of 8 per cent of 

 the population. 



With lens-type stereoscopes chromatic aberration occurs. In full-lens stereoscopes 

 in which the eyes are centered on the optical centers, white areas to the left of center 

 appear blue on the left side and red on the right. Areas to the left of center appear 

 the reverse. 



In split-lens stereoscopes white areas in the left-side view appear blue on the right 

 and red on the left. White areas in the right-side view appear the reverse. In the 

 fused image these somewhat cancel each other though a band of color remains. 



These effects are never bothersome, the edges of color generally being very fine and 

 only detectable if attention is directed to them. Such aberration can be corrected by 



the use of crown and flint glass in the stereo- 

 scope. In general the expense of doing this is 

 greater than warranted. 



The presence of residual color in anaglyphs 

 was mentioned under that subject. 



If the lenses of the stereoscope are exact 

 duplicates of those of the camera, no spherical 

 aberration will occur. If the curvature of the 

 lenses is different or if mirror stereoscopes are 

 used, such aberration does occur. It is exactly 

 the same problem as is present with an ordinary 

 single-lens camera and need not be discussed 

 here. 



Miscellaneous Applications. Astronomy. — 

 Stereography has certain limited uses in astron- 



T- 11 n a £ix J omy. All these are based on motion of the 



I'iG. 11. — Graflex camera fitted •' • i i i • i 



with two lenses for stereoscopic astronomical body with respect to the earth, 

 photography. Stereographs may be made of the moon, 



owing to its librations. These slight oscillations 

 are the same as a rotation of the moon. Consequently two views taken at opposite 

 ends of a libration cycle will show a stereoscopic effect. 



Stereographs may be made of astronomical bodies which move appreciably against 

 the background of the stars. This is done by photographing them twice so that they 

 appear shifted against the background. Aside from the fact that the object photo- 

 graphed, a comet or a planet, does stand out from the background in a stereoscopic 



