ILLUMINATION. 



39 



Stoney's heliostat can be purchased for five 

 pounds. It is a very reliable instrument and pos- 

 sesses the great advantage of only once reflecting 

 the incident ray. 



Artificial light has so many advantages over sun- 

 light that its employment is almost universal in 

 photo-micrography, Swan's electric lamp is the best 

 form, but as an ordinary paraffin lamp gives excellent 

 results, and the arrangements are similar for both, 

 we will confine ourselves to a description of the latter. 



Fig. 19 shows a simplified form of that used by 



FIG. 19. 



Dr. Dallinger and described in the Journal of the 

 Microscopical Society. 



To the back of an ordinary tin paraffin lamp A, a 



