4 GO A. C. BANFIELD ON A METHOD OF PREPARING 



other on the same baseboard. Each camera has its own lens, 

 the two lenses thus for photographic purposes replacing the 

 human eyes. 



I will now touch upon a point which is highly necessary to 

 secure correct stereoscopic relief — that of the separation of the 

 two lenses by which the pictures are taken. It has long been the 

 practice of the makers to supply stereoscopic cameras with the 

 lenses fixed at a distance apart of about 80 mm., a distance 

 which is absurd when we bear in mind that the average 

 interocular distance is no more than 62 mm. I imagine that 

 this excessive distance was adopted with the idea of getting 

 large pictures into each half, forgetful of the fact that shorter- 

 focus lenses would embrace the same view on a smaller scale and 

 yet enable the correct separation of 62 mm. to be observed. 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. I have also seen many stereo-photomicrographs in 

 which the same fault is evident. 



I have pointed out above the usual method 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 is a very simple arrangement, consisting 

 of a small board sliding in a grooved guide fixed to the tripod top, 

 the camera being fixed to the sliding board, to which an amount 

 of movement is allowed ecpaal 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 then changed and 

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

 taken : the two photographs resulting of course correspond to the 

 right and left eye view. Equally good results are obtained as in 

 the two-camera method, but moving figures of course cannot be 

 included in the photograph. 



It is by an inversion of this latter method that I prepared 

 the photographs wmich I have had the pleasure of showing 

 at this Club ; but before describing my method in detail, we 

 might consider the mathematical principles involved. As I have 

 already mentioned, we may accept the average interocular 



