OF THE MrcRoscorr.;. 145 



be preferred. But where a small light of great intensity is 

 needed, the common flat wick, turned edgewise to the mirror, 

 answers very well. It is a curious fact that flame is transparent 

 to light, and, therefore, the greater the depth of flame, the 

 more intense is the light. This is easily tested by looking at 

 the flame of a common hand lamp sidewise and edgewise. In 

 the latter case the eye receives the light from the entire flame 

 concentrated to a mere band. 



Several varieties of lamps have been devised specially for the 

 use of microscopists, and some of them are very excellent, 

 the most perfect being that devised by Dr. Drysdale and Kev. 

 W. H. Dallinger, and described in the Monthly Microscopical 

 Journal for April, 1876. 



It is hardly necessary to say that all kinds of oil have been 

 displaced by the mineral oils ordinarily called kerosene. 



Very intense light, such as that from burning magnesium, 

 the calcium light, the Bude light and others, have been tried, 

 but without material advantage. Many years ago, we arranged 

 a common kerosene lamp, so that the air surrounding the flame 

 could be enriched with a supply of pure oxygen when neces- 

 sary. Dr. Beale describes the same thing in his work, but does 

 not seem to regard it RH of any advantage. When used as a 

 source of direct light, however, we found that it more nearly 

 resembled sunlight than any other artificial source of illumina- 

 tion. A large diaphragm or shade, with an aperture of mod- 

 erate size, was placed close to the light, which was placed at 

 some distance from the microscope, and the rays passed di- 

 rectly through the object, not being reflected from a mirror. 

 The results in some cases were well worth the trouble incurred. 

 It is probable that in some cases very excellent results could be 

 obtained from the electric light if properly arranged. This, 

 however, is a department of microscopy which is certainly not 

 suited to beginners, and we, therefore, dismiss it. 



The rays of light, from whatever source obtained, are either 

 parallel, convergent or divergent; and in the illumination of 

 transparent objects the character of the light, as depending 

 upon these features, is of marked importance. This subject, 

 and the action of lenses and mirrors in changing the. relative 

 direction of the rays, should be carefully studied bv the stib 



