62 PHOTO-MICROGRAPHY AND PHOTO-MICEOMETRY 



sacrificed to pictorial effect, or a general or bird's eye view has 

 to be given, and the whole object crowded into the opening of 

 the largest mask ; but, surely, with the higher magnifications, 

 where the pictorial has given place to the diagrammatic, it would 

 be perfectly practical to adhere to recognised numbers, as, say, 

 125, 250, 500, 750, and 1000 diameters— perhaps the 750 might 

 be omitted. 



These numbers can easily be obtained, verified and recorded 

 by photography, for future reference. 



A stage micrometer is essential, preferably one cut to the 

 lOths and lOOths of a millimetre rather than to the lOOths and 

 lOOOths of an inch, as the subsequent calculations by the former 

 are much easier. I have six of these micrometers in my 

 possession, one by Zeiss, and the other five by London makers, 

 and I can strongly recommend the German slide as being 

 infinitely cleaner and truer cut than any of the others. 



This slide is then placed on the stage and exanimed in the 

 usual manner, care being taken that the microscope is at its 

 best. The following items should then be carefully noted and 

 recorded — whether the nose-piece is attached, and how this 

 affects the tube length — the condition of the draw-tube, and if 

 extended, the readings of its scale — the eye-piece, if any, 

 employed, and, if a projection one, the position of its spiral 

 focussing arrangement. The camera is now attached to the 

 microscope, and the joint made contact-tight, not uierely light- 

 tight. With a Hooke's key the picture is readily focussed on 

 the screen, and by using a metric rule and moving the camera 

 bellows to and fro, the required scale is speedily found. Say 1000 

 diameters are wanted. The 100th of a millimetre as cut on the 

 stage micrometer must be magnified on the ground glass equal 

 to 10 millimetres — that is, 1 centimetre. It is well to have a 

 scale on the camera to show the bellows extension. Most 

 cameras are so provided, but if not, it is a very simple matter to 

 attach to the base-board a yard tape, or a piece of one. Now 

 clamp the bellows and photograph the enlargement, and keep an 

 accurate record of the optical and mechanical arrangements, 

 including the total distance from the slide on the stage to the 

 ground glass screen, and it will be very easy on some other 

 occasion to adjust the apparatus for similar amplification. 



With my own appliances I obtain 1000 diameters by using 

 a 2 MM ( yo) apochromatic homogeneous immersion lens — short 

 or Continental tube, with nose-piece attached — draw tube opened 



