STEREOSCOPIC PHOTOGRAPHY 587 



If the latter method is used, care should be taken that the rotational alignment 

 is correct. One central point of both prints may be in vertical alignment, while one 

 print is slightly rotated with respect to the other. The eyes will allow an appreciable 

 rotation accompanied by a twisting distortion of the image without breaking fusion. 

 This can be avoided by watching the outside edges of the image, where fusion breaks 

 down with the least rotation while the center of the image remains fused. 



If possible, prints taken with a double-lens camera should not be cut apart until 

 ready for mounting as the distance between the views and the vertical alignment of 

 the prints is perfect in the uncut prints. By aligning corresponding points on the 

 prints, parallel to the base of the mounting card, with the prints centrally located, the 

 four corners of each view may be marked on the card. The prints can then be cut 

 apart and each mounted in the space formerly occupied by the other. 



Alignment of Transparencies. — Transparencies are made with exactly the same 

 materials of which lantern slides are made. They may be made by printing directly 

 from the uncut negative as follows: Place the left half of the transparency plate in con- 

 tact with the right half of the negative being sure that the edges are parallel. Shield 

 the halves not in contact and expose. Place the right half of the transparency in con- 

 tact with the left half of the negative. Shield the halves not in contact and expose. 



A simple frame can be made for this purpose composed of three sections each the 

 size of one view. The two end sections are shielded and the center one is open. 

 The negative and the transparency plate occupy two sections each, overlapping 

 in the center section. After exposure they are shifted to opposite ends and exposed 

 again. Zeiss manufactures such a frame. 



The value of this method is that, once the frame is properly constructed, the align- 

 ment and spacing of the resultant transparency is automatically correct. 



Alignment for Mirror Stereoscopes. — In mirror stereoscopes where large separate 

 prints are used, the prints can be cut to fit against stops in the stereoscope or they can 

 be aligned by observation. The latter requires a little knowledge on the part of the 

 observer of how to move the prints. 



Correct Vieioing. — The prints or transparencies being properly mounted must still 

 be properly inserted in the stereoscope and the latter held level. In showing stereo- 

 graphs to persons unfamiliar with them, fusion is frequently not achieved, either 

 because the slide is not level in the stereoscope or because the stereoscope is not held 

 level with the eyes. Both of these should be checked for persons having difficulty with 

 fusion. 



Adjustment of Stereoscope. — The viewing of stereographs after they have been 

 properly placed in the stereoscope is quite simple. Most lens-type stereographs have 

 focusing devices. It will be found easier to achieve fusion and involves less eyestrain 

 if the carriage or the lenses are brought forward to the proper position rather than 

 started forward and moved back. 



In mirror-type stereoscopes no focusing occurs. Views set in a particular position 

 may be made to fuse by changing the distance of the eyes from the mirrors. 



An effort should be made to obtain an even illumination over both views whether 

 prints or transparencies are used. The degree to which illumination may differ and 

 satisfactory fusion occur is certainly considerable. However, uneven illumination 

 makes the initial fusion a great deal more difficult and certainly detracts from the 

 resultant effect. 



Stereoscopes for both prints and transparencies can be equipped with a light 

 mounted in such a way as to give an even illumination. 



When a light is used on prints care must be taken to avoid glare. As a glossy piint 

 makes a more satisfactory background than a dull one, this glare can be considerable. 

 It is sometimes eliminated by warping the prints into a slight curve. 



