Illuminating Apparatus. By J. W. Gordon. 429 



We thus obtain a light source which is structureless, and which, 

 as we have already seen, can be made to vary indefinitely in 

 intensity. 



The rest of the apparatus can be very briefly described. To 

 the polished end of the speculum are fitted stops of various forms 

 and sizes for the purpose of giving any required shape and 

 dimensions to the light source. The fittings in which the burner 

 and speculum are carried are made adjustable in height for the 

 purpose of collimation, and the whole is placed upon a convenient 

 stand adapted to hold the various parts in due relation to one 

 another, while permitting the requisite freedom of motion. 



It will, of course, be understood that this speculum can be used 

 not only with a Nernst lamp, but with any form of illuminant. 

 Filamentous lamps, like the Osram and Tantalum lamps, which 

 burn with very high brilliancy, are equally available, although, as 

 these lamps are ordinarily made, their filaments, being inclosed in 

 a glass envelope, cannot be brought into such close proximity to 

 the ground-glass end of the speculum as the Nernst lamp, which 

 burns in the open. Whatever the form of the primary source of 

 light, the speculum pattern retains its even illumination and 

 symmetrical figure, with the result that, whether focused in the 

 plane of the object — that is to say, under the conditions of what is 

 commonly called " critical illumination " — or not, the diffracting 

 aperture is of circular form, and therefore yields a symmetrical 

 antipoint, producing the best image which any Microscope with 

 which it may be used is capable of yielding. 



