MICROSCOPIC OBJECTS. 



13 



as possible, consistent with suitable illu- 

 mination. The writer frequently uses a 

 cardboard screen between the lamp and mi- 

 croscope, with an aperture to allow the 

 rays to pass through. By this means the 

 microscope is kept cool, and possible injury 

 to the lens and object averted. The alum 

 cell, used with the heliostat to stop the 

 heat rays, can hardly be used successfully 

 with lamplight. Sometimes, however, a 

 thin cell, containing ammonio-sulphate of 

 copper, may be employed when photo- 



graphing very transparent diatoms, and an- 

 swers the double object of keeping off the 

 heat from the lamp and giving a more dif- 

 fused light. For the latter purpose, with 

 low powers only, a strip of fine ground 

 glass or oiled paper may be placed beneath 

 the object. A slide of blue glass, 3 inches 

 by I inch, is also used by some operators 

 for softening the light. 



All being ready for an exposure a black- 

 ened card must be placed opposite the lens, 

 or below the stage, to cut off the light 





e 







0 











a, Magnesium lamp ; b, condensers ; c, microscope ; d, focussing rod attached to fine adjustment ; e, 

 support of microscope ; /, support of focussing-screen^, and moving backwards and forwards in a line with 

 the microscope, either in guides or on rails ; g, focussing-screen. 



With a low power, the card should be in 

 front ot the lens. The dark slide may then 

 be inserted, and the shutter drawn up. Af- 

 ter waiting for a few moments to allow all 

 vibration to cease, the card in front of the 

 lens must be rapidly removed. During the 

 exposure the operator must abstain from 

 walking about the room, for the vibration 

 so produced would injure the sharpness of 

 the picture, more especially with low pow- 

 ers and short exposures. When the expos- 

 ure is deemed sufificient, the blackened card 



must be replaced in front of the lens, and 

 the shutter pushed down. If other expos- 

 ures are to be given, do not turn the lamp 

 down, but leave the flame the full height, 

 until the next plate can be inserted in the 

 camera. In this way the alternate expan- 

 sion and contraction of the microscope, etc., 

 is avoided. If, on developing, the first 

 negative be found either over or under-ex- 

 posed, try again, without disturbing the ap- 

 paratus. 



This and the former figure illustrate the 



