STEREOSCOPIC PHOTOGRAPHY 581 



It is also possible to fuse stereographs by looking at them cross-eyed. This pro- 

 duces a small image in front of the plate which is psuedoscopic — reversed in depth. 

 This method can be employed on stereographs before they are cut apart and trans- 

 posed, in which case they will not be psuedoscopic. 



Lens Stereoscopes. — The usual type of stereoscope for general work is the lens 

 stereoscope. The purpose of the lenses is primarily to relax the accommodation. In 

 split-lens stereoscopes the views are also partiallj^ drawn together by refraction. 



The lenses thus serve to keep the ej^es focused on infinity which is approximated in 

 normal vision by all objects more than a few feet away. Thus the normal accommoda- 

 tion-convergence ratio is not too seriously violated as it is when the eyes are forced to 

 remain focused at the distance of the plate. 



Full-lens Type.- — In stereoscopes of short focal length, full lenses are usual Ij- used 

 separated by 2J-^ in. They are available as box or as folding types. 



The box type is used for transparencies. It is a simple box with a ground-glass 

 screen in one end and the lenses mounted in the other end. The lenses are usually 

 fitted with a rack-and-pinion focusing device much after the manner of a pair of opera 

 glasses. The plate is inserted through a slot in front of the glass screen. The latter 

 serves to diffuse the light over the entire scene. The instrument must be pointed at a 

 satisfactory light source unless it is fitted with a light in back of the glass screen. 



The folding type is convenient for carrying in the pocket for use with prints. It 

 consists of a collapsible metal framework containing the lenses and a rack for the 

 plate. These may be constructed of two pieces which slide in each other in order to 

 permit focusing; or they may be of one piece with a fixed focus. 



The value of this type of stereoscope is that it can be made very cheaply and is of a 

 convenient size for carrying. However, the cheaper forms lack rigidity and easily 

 get out of alignment. 



Spectacle Type. — Pairs of lenses much like a pair of spectacles are also obtainable. 

 These require that the plate be held in the hand and moved until fusion is made. This 

 requires a certain amount of practice. This type is very convenient for use with books 

 in which stereographs are published. 



The Camera as a Stereoscope. — For theoretically perfect orthostereoscopic viewing 

 the stereograph should be viewed with the pair of lenses with which the pictures were 

 taken. Some cameras on the market are so constructed that they may also be used 

 as viewers. However any pair of lenses of the same focal length and a comfortable 

 diameter will give excellent results. 



Split-lens Type. — The common hand type, or Brewster stereoscope, the American 

 parlor stereoscope, is of the split-lens type. This system is shown in Fig. 5. The 

 single lens is split vertically through the center and the two halves are mounted with 

 the optical centers out. These centers are separated somewhat in excess of the inter- 

 ocular distance. (Keystone telebinocular, 95 mm.) This allows wider separation 

 of the prints and consequently larger prints may be used without overlapping. 



The alteration of depth of the objects in the stereograph in this type of stereoscope 

 when used with a camera of interocular separation is nOt detectable in the ordinary 

 photograph. It need only be given serious consideration in technical work where the 

 accurate judgment of depth relative to width and height is of importance. In such 

 cases the use of a camera of lens separation equal to the distance between optical 

 centers in the stereoscope eliminates the distortion though the image is theoretically 

 nearer and smaller on hyperstereoscopic principles. 



In general this type of stereoscope has a longer focal length than the full-lens type. 

 The difficulty here is that the focal length is usually longer than that of most cameras. 

 On the other hand the splitting of the lens allows of a larger and consequently flatter 

 full lens. This reduces both chromatic and spherical aberration. 



