1894.] MICROSCOPICAL JOURNAL. 385 



Mr. Cunningham's Method of Illumination. 



BY DR. EDWARD GRAY, 

 SAN FRAXCISCO, CAL. 



Mr. Cuiiiiingliam's article, in the August issue of the 

 Journal proposes the thesis that the diatom is a Proto- 

 zoan, an animal therefore, and as proof of the assertion 

 he supplies a method of demonstration. Of this he says, 

 at page 231, "an easy method of verification [i. e., of 

 the animal nature of the living diatom] is accessible to 

 all who use the microscope as an instrument of research 

 or for biological studies of any kind." The present ar- 

 ticle proposes to deal not with the main proposition, but 

 with the kind of evidence upon which it rests. So radi- 

 cal a proposition as is advanced by the Mobile diatomist 

 demands unimpeaceable evidence of its truth. Is Mr. 

 Cunningham's evidence of such character ? Let us see. 

 His method of illumination supplying the demonstration 

 is thus described (page 234 :) "In regard to the light- 

 ing, and some other requisites of illumination, an Ar- 

 gand, burner lamp is used, a bull's-eye condenser being 

 adjusted as near as possible to the flame, and a large im- 

 age of the flame projected so as to fall upon the concave 

 face of the mirror. To the sub-stage an achromatic con- 

 denser is adapted, and when the light is properly cen- 

 tered in the field, the result will be a dazzling light. 

 " But in order to guarrantee the successful view of the 

 various phenomena, it is necessary to have at hand a 

 glass slip, or a smaller piece of emerald or grass-green 

 colored glass (blue will not answer). This slip must be 

 placed on top of the condenser or the slide contain- 

 ing the living diatoms must rest directly upon the green 

 glass slip." 



It is surely a matter of regret that Mr. Cunningham 

 should have left unexplained why blue glass will not an- 

 swer, while green glass meets the conditious. The wave 



