362 THE president's address. 



Note, it is not necessary that the microscope and camera should 

 be in the centre of the board, they can be placed close to the 

 edge on the side you are working, a very slight bend over is 

 then required to place the eye in the optic axis, as it is only 3^ 

 inches from the edge of the board ; but the optic axis in the 

 turn out microscopes, with 12 X 10 cameras fixed in the centre 

 of the board, is probably three times that distance from the 

 edge. 



Below the board are two taps, one of which is fitted with a 

 bye pass ; this is of great use because when the jet taps have 

 been once adjusted, the light can be turned on and off without 

 disturbing them. The bye pass has also a tap, so that when it 

 is in use the light can be made a mere pin's point ; this, of course, 

 saves gas. The advantage of having the bye pass separate from 

 the lamp is obvious ; you can turn the light up and down as 

 many times as you please without the slightest risk of disturbing 

 the centring or any of the adjustments. 



With regard to the gas it is self-evident that if a pressure 

 equal to that usually employed in an optical lantern is used, 

 the size of the incandescent lime will be too large, and the 

 light too brilliant for our work ; the pressure, therefore, must 

 be reduced. 



It is, in some respects, not so easy to maintain a steady light 

 with reduced pressures as it is with high pressures. A pressure 

 equal to two inches of water is employed ; this is obtained by 

 using ordinary gas holders of five cubic feet capacity each. The 

 oxygen one is charged from a cylinder, and the hydrogen from 

 a gas bag, which has been filled from the main. The amount of 

 gas consumed is small, probably not exceeding IJ cubic feet 

 per hour ; this, however, is by estimation, not by measurement. 

 The last and simplest, though important, part of a photo- 

 micrographer's outfit is a piece of white card ; this should be in 

 constant use, because by it the evenness of the illumination can 

 at once be determined. This is its most important office, but 

 it will be found useful in many other ways. For example, 

 when working with a long camera composed of several tubes 

 that awkward operation with ordinary apparatus, viz., the 

 arrangement of the image at the far end, is simplicity itself, for 

 it is only necessary to separate two tubes near the microscope 

 end, when the state of the image at that point can be examined 



