Aet. JjTL— Memorandum on the Surirella gemma; bj J. J. 

 Woodward, Assistant Surgeon, U. S. Army. 



The Surirella gemma has been recommended b.y Hartnack 

 as a test for immersion objectives of bigb povvei-s. ' I have not 

 gamed access to his original description, but find accounts of 

 his views, with figures, in the works of Drs. Carpenter and 

 l^rey. (<' The Microscope and its Eevelations," 4th edition, p. 

 182. "Das Mikroskop," 3d edition, p. 40.) Hartnack ob- 

 served fine longitudinal striae in addition to the fine 



ones previously known to exist between the large 

 nbs ; he supposed the true markings to have the form of elon- 

 gated hexagons. 



Two handsome slides of this diatom were received at the 

 Army Medical Museum a few months since, from Bourgogne 

 ot Pans. A careful study of these by monochromatic sunlight 

 inchnes me to the opinion that Hartnack's interpretation is er- 

 roneous, and that the fine strise are in reality rows of minute 

 Jf^^isphencal bosses ; from which, as in the case of other di- 

 o+ xr . arance of hexagons would readily result if the 



- vvds uuserved by an objective of inferior defining power 

 I used, or if the illumination was unsuitable. This 



memorandum is accompanied by two photographs exhibiting 

 ^hat I saw ; one is magnified 1,034, the other 3,100 diameters. 

 ihe pnncipal frustule shown in these photographs is ^i^th of 

 an inch m length. (The mean length of S. gemma is stated 

 ^J the Microscopic Dictionary at ^i^th of an inch.) The 

 the^ ^T^^^"^^ striae counted longitudinally at the rate of 72 to 

 1 /"/" ^^ ^^ ^^^^ Transversely these were resolved into 

 Qf^®^ appearances which counted laterally 84 to the ToVoth 

 finp i!^^ • ^^ *^® structure consists, as I suppose it does, of 

 ^"e uemisptierical bosses, proiecting from the surface of the 

 J^ustuies, the fact that these bosses are set together more closely 

 aoi- ^J^^sverse direction than in the longitudinal would 

 ^ount for the elongated form of the pseudo-hexagons when 



deE*?!-^^^^^ ^^ *^® photographs closely approach Hartnack's 

 ^nption, but it is easy to observe that these are not the parts 

 ^^ich are most nearly in focus. 



deri T^^? resolved this diatom by monochromatic light 



^aZ^ ^^'^ *^e electric lamp. The appearances obtained 

 Z^ Identical with those above described. 

 wPKS,?"'-^^'^^ 5«?«^«»'« Office. 



