180 PEACTICAL PHOTO-MICKOGKAPHY 



consistency. This should be rubbed over any part of the wood- 

 work, and will effectually blacken any reflecting parts. In the 

 case of the inside of the bellows of the camera not being dead 

 black, it is best to make a mixture of lamp-black and ordinary 

 flour paste, and apply this where required. 



It will be found that it is a good plan to interpose dead- 

 black metal diaphragms with suitable openings between the 

 camera front and the ground-glass focussing-screen. Many 

 makers supply these thin metal plates with openings of various 

 sizes which fit into the back of the camera, immediately in front 

 of the position taken by the dark-slide. It is advisable to use 

 one with as small an opening as possible when photographing. 

 Only harm is done by allowing useless portions of the image 

 to fall on the photographic plate, since they are then diffusing 

 unnecessary light inside the camera body. If the camera 

 bellows are in segments, a metal diaphragm may be usefully 

 interposed about midway between the camera front and the 

 photographic plate. 



Heat from Illuminant. With most high-power illuminants 

 some method of absorbing the radiant heat is necessary. This 

 is particularly the case with the electric arc and lime-light, and 

 when either of these is in use it is advisable to ensure that no 

 unnecessary heat reaches the microscope. Otherwise expansion 

 of the components is sure to take place, and sufficient disloca- 

 tion will ensue to alter the focus during an exposure of any 

 considerable length, with the result that the image is not well 

 denned. This may be prevented by placing a water-bath in 

 as near a position as convenient to the source of light. Such 

 an appliance is sold by most opticians ready made, but an 

 improvised one may be constructed in the manner described for 

 making cells to contain coloured fluids, referred to in a later 

 chapter on this subject. The cell should be filled with well- 

 boiled distilled water, to ensure that all the air is driven off; 

 otherwise the heat of the radiant will be sufficient to cause air- 

 bubbles to appear on the walls of the containing- vessel. It is 

 sometimes advised that the absorbing fluid should be a saturated 

 solution of alum instead of water only, but this would not 

 appear to increase the heat-absorbing power very greatly. In 

 cases where a yellow screen is used, then a weak solution of 

 perchloride of iron may be substituted for plain water, and will 



