31 



of the lenses for that particular magnification. Thus, if we are 

 photographing an object x 62, then the objectives must be 

 separated 1 mm. ; if x 31, then the separation would be 2 mm. 

 This rule holds good whatever type of objective is used and for 

 all magnifications ; at 1,000 diameters the separation is 62 micra. 

 These considerations lead at once to the root of the subject, as 

 it will be realised that no objectives made could be placed so close 

 together as 1 mm. It might, perhaps, be possible to get them, 

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

 could then work to x 6 ; but such a system would be far too 

 inelastic for all-round work. We should want to take photo- 

 graphs at much higher and also lower magnifications, so other 

 means must be sought. We find that the single-camera method 

 referred to is suitable in every respect ; there is only one lens 

 to consider, necessitating two separate exposures ; also the method 

 enables us to work at any desired magnification. It is, of course, 

 impracticable to move a long and heavy camera, such as is 

 necessary in photomicrographic work, the often minute distance 

 required for the objective separation ; but it is very easy to move 

 the object we wish to photograph any required distance, great or 

 small, by a suitable mechanical arrangement. Mr. Banfield then 

 proceeded to describe in detail the apparatus he employed for 

 this work, and which he had on exhibition at the meeting. 

 It consisted of two of the well-known Zeiss optical benches 

 placed end to end, mounted on heavy trestles. For low mag- 

 nification (up to about X 10) one only gives sufficient extension. 

 The lecturer i-emarked on the exceeding usefulness of these 

 benches. At one end of the bench is fixed the lamp-casing, the 

 bench itself carrying the condensers, object- stage, objective and 

 camera, all of these adjustable in any position on the bench. 

 The camera employed was adapted to the English standard 

 stereoscopic size, 6| x 3| in. The formula mentioned with regard 

 to objective separation resolves itself in practice into two parallel 

 lines drawn on the focussing screen 62 mm. apart. By means 

 of the stage the object is moved till one of the lines cuts the 

 image centrally the first exposure is made. The image is then 

 transferred to the other line, when a second exposure " gives a 

 truly stereoscopic pair. The objectives usually employed were 

 Zeiss " Planar," of 20 and 35-mm. focus, and one of a similar 

 type by Leitz, of 42-mm. focus. The last named was found 



