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impressions of an object received by each of our eyes, owing to 

 their angular separation. It is entirely due to the separation, 

 of our eyes that we are enabled to so readily locate the position 

 of an object in space with reference to other objects. In practice, 

 stereoscopic photographs of ordinary objects, views, etc., are 

 taken by means of two separate camei-as, for convenience 

 mounted parallel to each other on the same baseboard. Each 

 camera has its own lens, the two lenses thus for photographic 

 purposes replacing the human eyes. With reference to the 

 separation of the two lenses, it was stated that it was the 

 practice of makers to supply stereoscopic cameras with lenses 

 tixed at a distance apart of about 80 mm., a distance which is 

 absurd when it is remembered that the average interocular 

 distance is about 62 mm. The result of this excessive but usual 

 separation is very apparent in the distorted and uncanny sense 

 of relief experienced in looking at the " commercial " stereoscopic 

 views, which are on sale everywhere. The same fault is evident 

 in many stereo-photomicrographs. The two-camera method is 

 the usual way of preparing stereoscopic photographs, but there 

 is another way by which they can be made, though not with 

 the .same facility that is, by the use of a single camera. At 

 many photographic dealers a small fitting can be obtained, which 

 is placed between the camera and the tripod top. It consists of 

 a small board sliding in a grooved guide fixed to the camera-, 

 tripod top. The camera is fixed to the sliding board, and an 

 amount of movement allowed equal to the usual erroneous ocular 

 separation of 80 mm. In practice, a photograph is taken with 

 the camera at one end of the slide, the plate is changed and the 

 camera moved to the other end, when the second picture is taken, 

 the two, of course, corresponding to the right- and left-hand 

 view. Equally good results are obtained as in the two-camera 

 method ; but moving figures cannot, of course, be included. The 

 lecturer said that it was by an inversion of the latter method 

 that the photographs had been prepared which he had the 

 pleasure of showing that evening. But before describing his 

 method in detail, he would say a few words on the mathematical 

 principles involved. He accepted the average interocular distance 

 as 62 mm., and this distance furnished the base for practical 

 photomicrographic work, as the quotient obtained by dividing this 

 distance by the desired magnification gives the correct separation 



