332 THE AMERICAN MONTHLY [Dec, 



no perspective and the stereoscopic effect produced by the use 

 of one-half the objective is due, not to differing lateral views of 

 the object, but to the lateral displacement of the images of con- 

 secutive layers effected by the oblique pencils employed. In 

 effect the lateral displacement of the images of successive lay- 

 ers is similiar to that produced by differing Literal views of the 

 objef't, and pictures so obtained are not only stereoscopic but 

 they are true representations of the object viewed. The utili- 

 zation of one-half theobjective is similar to the i)rocess employed 

 for obtaining stereoscopic effect in binocular microscopic vision, 

 where, by the use of a prism in the tube of the microscope, the 

 rays from the right half of the objective are projected to the left 

 eye and the rays from the left half are sent to the right eye. 



When using this method for stereoscopic photomicrography, 

 either one-half the front of the objective or one-half the back is 

 covered, but in either case the covering must fit close against 

 the glass of the objective. A cap covering one-half the front of 

 the objective may be fitted to objectives up to one-fifth inch 

 focus, but objectives of higher power can not be capped in this 

 way as their working distance is too short. With low power 

 objectives, in fact with all powers which have sufficient working 

 distance to allow focussing after tilling the slide, it is best not 

 to cap the objective but to use the full objective and get the de- 

 sired stereoscopic effect by tilting the side. In this way the 

 objective is worked to its best advantage and the lateral views 

 of the object are real and not due to optical displacement of its 

 different planes. Also, when the lens is capped, it is impossible 

 to even illuminate a large field and this is the great disadvan- 

 tage as it restricts one to the use of a small central field. In 

 tilting the sHde, the axis oj the tilt must be parallel with the upright 

 axis of the object, and this axis must be maintained when the 

 pictures are mounted on the stereoscopic card. That is, if the 

 object photographed is an insect so mounted that its head points 

 upward when the microscope is horizontal for photography, then 

 the slide must be tilted from the end and the photographs of 

 the insect must be mounted with the head of the insect toward 

 either the top or bottom of the card. If, however, the insect is 

 lengthwise of the slide, the slide mu^t be tilted from the side if 

 the picture of the insect is to be mounted head up or down. It 

 is not strictly necessary to tilt the object for both exposures and 



