GENEEAL PEELIMINAEY PEEPAEATIONS 181 



much more effectively absorb the heat rays. The greater the 

 heat emitted by the radiant, the larger the water-bath should 

 be, so that there is sufficient water to prevent any considerable 

 rise of temperature and consequent injury to the objectives 

 during a long exposure. In any case the cell should be at 

 least one inch in thickness. 



Diaphragms. It is convenient to interpose between the 

 source of light and the microscope one, and in some cases two, 

 diaphragms, either of the iris pattern or of the type in which 

 a series of metal plates with openings of various sizes can be 

 inserted at will. It is usual to have one of these as close to the 

 illuminant as possible. In the case of illuminants of rather large 

 area such as the mercury- vapour lamp one may be placed 

 almost in contact with the tube of the lamp itself. It is well 

 that any part of the illuminant that is not required in actual 

 use should be effectually screened off. In cases where a system 

 of collecting-lenses such as the Zeiss-Kohler lenses already 

 described are being used, then a diaphragm may also be 

 interposed between the two lenses. It is, with any system 

 of illumination, advisable for the illuminating beam to be con- 

 tracted to such a size that no portion of the beam, other than that 

 actually in use, is allowed to reach the microscope. In any case 

 where one or two diaphragms are used the position should be 

 chosen so that any extraneous or unused light is effectually 

 screened off. The iris-pattern diaphragms are obviously the 

 most convenient, but the cheaper metal plates with suitable 

 sized openings are perfectly efficient. 



Size of Camera. This is a point actually of no particular 

 importance, but where a camera is being obtained especially 

 for the purpose in view, it is advisable that its size should not 

 be greater than is really necessary. In the large majority of 

 cases what is known as a quarter-plate camera is sufficient, as in 

 any other than low-power work the area of good definition will 

 rarely exceed that of a circle of three inches ; consequently, 

 the plate of 3J r/ x 4J" (quarter-plate) will be sufficient for this 

 purpose. 



When using Planar or similar low-power lenses, however, 

 the area of good definition is considerably larger, so that it may in 

 general be more convenient to adopt a half -plate size, 6J" x 4". 

 This will entail the use of plate- carriers when using the plates of 



