76 



PKACTICAL PHOTO-MICKOGKAPHY 



so that, broadly speaking, the more powerful the source of light 

 which it is possible to use, the better the result that may be 

 expected. It is also stated by no less an authority than 

 Professor Abbe that the more powerful the source of light, the 

 better the resulting optical image. Powerful illuminants are 

 usually small in area, so that, both on theoretical and practical 

 grounds, they are desirable, since they approach more closely 



to that great desideratum, a point of 

 light. On the other hand, for low- 

 power work, and particularly in cases 

 where the objects are thin or trans- 

 parent and lacking in contrast, a 

 less powerful light will fulfil every 

 requirement and may even give the 

 best result. 



Oil Lamps. Probably most 

 readers will at least commence work 

 with an oil lamp, and if the length 

 of exposure necessary as the result 

 of using this is no particular ob- 

 jection, then a good oil lamp, care- 

 fully chosen and well constructed, 

 will answer many purposes. The 

 lamp should be either of the type in 

 which the wick is broad and flat, or 

 with a circular burner of the Argand 

 pattern. If of the former variety 

 it should have a wick at least one 

 inch broad and should be so arranged 

 that the lamp can be turned either 

 with the broad side or the edge of 

 the flame towards the microscope. 



The usual type of microscope lamp as sold by opticians is 

 as a rule not the most suitable, for they are generally small 

 in the body, and altogether not solid or substantial enough 

 in construction. One should be chosen with a vertical 

 adjustment for centreing the light. This is usually ob- 

 tained by carrying the oil reservoir on a bracket which slides 

 up and down on a vertical rod rising from the stand, and 

 which can be clamped in any position (Fig. 22). The writer, 



FIG. 22. Oil lamp. (C. Baker.] 



