1894.] 3IICR0SC0PICAL JOURNAL. 385 



Mr. Cunningham's Method of Illumination. 



, BY DR. EDWARD GRAY, 



S\N FKANCISCO, CAL. 



Mr. Cunningham'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 conditions. The wave 



