GENEEAL PKELIMINAEY PKEPAKATIONS 179 



is cheap and easy to construct, and works well under all 

 conditions. Further, there is no actual contact between 

 the microscope tube and the front of the camera itself. Let 

 A A represent the camera front, and BB an open tube extending 

 from its centre. This tube BB may be of such a size that it 

 can be conveniently used for putting on a carrier for a micro- 

 scope objective of low power, or an ordinary photographic 

 lens for the photographing of culture-tubes or similar objects. 

 On the ocular end of the microscope body-tube a double tube 

 CO is placed ; the inner and smaller tube is of such a size that 

 it just slides over the body-tube, while the outer tube is suf- 

 ficiently large to have at least one-eighth of an inch clearance 

 outside the tube BB of the camera. When the camera front 

 is therefore pushed up so that its tube lies between the double 

 tube of the fitting CO, no extraneous light can enter between the 

 microscope and camera. The whole of this appliance should 

 be dead black to prevent the possibility of reflections from any 

 of the exposed surfaces. 



Diffused Light. It is advisable to protect as far as possible 

 the whole of the apparatus from diffused light, whether from 

 the illuminant or from any extraneous source. The illuminant 

 should therefore be enclosed in a suitable casing with an 

 opening, other than any necessary for the purpose of ventilation, 

 only in the direction of the optical system. Some workers also 

 advise that the whole of the optical system should be covered 

 with a black-cloth hood to prevent the possibility of stray or 

 diffused light entering the apparatus. While this may be an 

 excellent arrangement in cases where the work is carried on in 

 a well-lighted room, yet in any special apartment where other 

 bright sources of light are not in use, it would not appear to be 

 necessary. 



It is highly important to see that there are no disturbing 

 internal reflections in any part of the apparatus. In well-made 

 microscopes this is not likely to be a source of trouble, but inside 

 the camera itself it is by no means unusual to find that there 

 are some surfaces that are diffusing or reflecting a considerable 

 amount of light. If this arises from the apparatus not being 

 dead black inside, it should be immediately rectified. This 

 can be done by taking some dilute shellac varnish and adding 

 to it sufficient fine lamp-black to form a mixture of a creamy 



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