578 A. A. C. ELIOT MERLIN ON DIATOM STRUCTURE. 



examined with dark-ground illumination and a rather small stop. 

 This led me to search my Coscinodiscus genus circle slide for 

 the diatom mentioned. Although this could not be found, another 

 form was noted appearing with transmitted light as in photo- 

 graph No. 2 (x 295), while with dark-ground illumination a 

 beautiful radiating structure, somewhat resembling A. helio- 

 pelta, was revealed, which photograph No. 3, taken at the same 

 magnification, inadequately represents. Print No. 2 fails to 

 show a fine dotted structure which exists all over the valve 

 and can be detected in parts of No. 3. Print No. 2 should make 

 the identification of this specimen easy from its very marked 

 peculiarities.* These photographs were taken with a 16-mm. 

 apochromat of 0'35 N.A. and a x 6 projection eyepiece. 



In connection with diatoms as test-objects there is an interest- 

 ing point, to me at least, on which I have been able to find no 

 definite information in the microscopical works in my possession. 

 I am alluding to the exact period during which the fine structure 

 of the diatom valve was first employed as a test-object. Are 

 we approaching the centenary of its discovery, a discovery 

 which has perhaps influenced more than any other the progress 

 towards perfection of the modern microscope stand and its 

 optical parts ? The oldest work on the microscope in my library 

 is the Microgr aphia Restaur ata, published in 1745.| This makes 

 no mention of diatoms, so that it may be taken for granted 

 that Dr. Hooke did not include " diatom-dotting " amongst his 

 ' ' Wonderful Disco veries by the Microscope ' ' therein detailed. George 

 Adams published the fourth edition of his Micrographia Illustrata 

 in 1771 and also failed to include diatoms amongst the numerous 

 objects described in his interesting book, although many quaint 

 aquatic organisms are dealt with at considerable length, even 

 including ' a new sort of animalcula found in an infusion of 



* C. Ludovicianus (Rattray) from Jutland. 



f Dr. Robert Hooke, M. A., F.R.S. (1635-1703), Micrographia,or some 

 Physiological Descriptions of Minute Bodies made by Magnifying 

 Glasses, with Observations and Inquiries thereupon. The first edition 

 was published in London, 1665. 



