430 Transactions of the Society. 



XIV. — Corethron criophilum Castr. 

 By Edward M. Nelson. 



(Read June 11th, 1908.) 



During the examination of a strewn slide this diatom was found. 

 Critical examination showed that the " awns," or bristles of the 

 " broom," had minute secondary spines arranged spirally round 

 them. These secondary spines, though quite conspicuous in other 

 varieties of this and kindred species, have not been previously 

 observed on this variety. These secondary spines are not like the 

 short thorns upon a Bacteriastrum* but they more resemble the 

 spines upon the hair of a Polyxenus Lagurus. They are minute, 

 being much about the size of a fiagellum of a bacterium. 



It is not on account of the insignificant biological importance 

 of the discovery of these small secondary spines that this note is 

 brought before you this evening — nevertheless, it is as well that 

 an organism should be figured as correctly as possible— but from a 

 microscopist's point of view this diatomic structure does possess 

 some importance, for not only is it an excellent test object, but it 

 will, better than almost any other object, enable a microscopist to 

 discriminate between the various competing methods of illumina- 

 tion. There can in this case be no doubt about the structure : no 

 question can possibly arise as to whether it is a hole or a boss, a 

 puncta or a pearl. 



In this object the focus for the white dot image is longer than 

 that for the black dot. The thickness of the " awns "is • 000006 

 inch plus 0-000003 for antipoint correction = 0-000009 inch, 

 or 0*23 fM. It is then an interesting question if this structure can 

 be better seen with a small or large axial cone of transmitted light, 

 or upon a dark ground obtained, by the lately revived oil immer- 

 sion paraboloid, by Mr. Gordon's top-stop arrangement,or by some 

 other method. 



Apart from these considerations, questions upon the theory of 

 microscopic vision are opened up, fOr at the present time it is held 

 that a self-luminous bright line of great tenuity can be seen upon 

 a dark ground where a dark line of the same thickness upon a 

 bright ground could not ; but Mr. Gordon will tell you whether an 

 object, such as this particular structure, illuminated upon a dark 

 ground, would behave as a self-luminous object or not. 



* Journ. Quekett Micr. Club, iii. ser. 2, pi. 4, fig. 2, p. 42. 



