STEREO-PHOTOMICROGRAPHS. 461 



-distance as 62 mm. and this distance furnishes us with a basis for 

 practical work, as the quotient obtained by dividing this distance 

 by the desired magnification gives us the correct separation of 

 the ' lenses for that particular magnification. Thus if we are 

 photographing an object x 62, then our objectives must be 

 •separated by 1 mm ; if x 31, then the separation must be 2 mm. 

 This rule holds good whatever type of objective we use and 

 at whatever magnification. If we wish to obtain a correct 

 •stereoscopic representation of our object at 1,000 diameters, the 

 objective separation becomes 62 micra. 



This consideration brings us at once to the root of the subject, 

 as it is at once realised that no objectives yet made could be 

 placed as close together as 1 mm. We might possibly get them, 

 say, 10 mm. in diameter and by mounting them close together 

 we could then work to x 6, but a system such as this would be 

 far too inelastic for all-round work ; we should occasionally want 

 to take photographs at much higher and also lower magni- 

 fication than this, so we must seek other means to fulfil our 

 purpose. 



Let us turn to the single-camera method referred to above. 

 This we at once find suitable in every respect. We have only 

 one lens to consider, which, although rendering two separate 

 exposures necessary, enables us to work at any magnification we 

 wish. 



It is of course impracticable to move a long and heavy camera, 

 such as is necessary in photomicro work, the often minute 

 -distance required for the objective separation, but it is very easy 

 to move the object which we wish to photograph any required 

 •distance, great or small, by suitable mechanical arrangement. 



I will now describe in detail the apparatus which I have used. 

 It consists of two of the well-known Zeiss optical benches, mounted 

 ■on trestles. For very low magnification (to about x 10) one only 

 is used ; for higher magnification than this they are placed end 

 to end. I may remark here, in parenthesis, on the exceeding 

 usefulness of these optical benches. There is indeed no optical 

 experiment that cannot be performed on them. 



At one end of the bench is fixed the lamp casing, the bench 

 itself carrying the condensers, object stage, lens and camera, all 

 of them adjustable in any position on the bench. The camera 

 itself is a very simple affair, adapted for the English standard 



