78 
PRACTICAL PHOTO-MICROGRAPHY, 
is a grave consideration, not so much for economy of 
time, as for lessening the danger of tremor, and of the 
parts of the microscope being displaced during exposure 
by changes of temperature and other causes. Roughly 
speaking, a good oil lamp will entail about thirty times 
as much exposure as a good lime-light. 
Fig. 25. 
If oil light is to be used it is well to have it as 
powerful as possible; the ordinary lamps used in micro¬ 
scopical observation are by no means powerful enough. 
Our lamp should have a wick at least one inch wide, 
