DIATOMACE.E. 81 



thicker portions, which project like teeth, strongly 

 contrast with the difficultly distinguishable portions 

 of the thin interspaces. The fossil forms from the 

 Bermuda deposit are best for the investigation of 

 this structure ; many of these are extremely beautiful 

 microscopic objects, their markings resembling those 

 on the engine-turned back of a watch. 



The detection of the finer markings of the Diato- 

 macese, which, according to my view, consist of de- 

 pressions like those upon the valves of Coscinodiscus t 

 is a matter of great difficulty to those who are un- 

 accustomed to the use of the microscope, and who 

 have not a complete set of apparatus. The main 

 point to be attended to in bringing them into view, 

 is to use one-sided oblique light, i.e. to turn the 

 mirror by its stem as much as possible to one side, 

 and then to incline it so as to throw the light upon 

 the object. In this way the valves of the species of 

 Gyrosigma, for instance, appear covered with lines 

 (PL VI. figs. 16 and 17) ; but the lines are spurious, 

 i. e. they are the optical expression of rows of minute 

 dots (figs. 16 0, 17 a) ; and when oblique light is 

 thrown upon the valves from all sides, by means of a 

 special achromatic condenser, in which the central rays 

 are excluded, the dots become distinct, and the mark- 

 ings resemble those on the valves of Coscinodiscus. To 

 show the finer dots clearly, a valve should be crushed, 

 so as to obtain a fragment as flat as possible ; for the 

 surface of the valves is curved more or less in all the 

 species. The valves of G. angulatum are generally 

 used to test the quality of the object-glasses of the 

 microscope, and also for practice in " making out " 

 the lines and dots ; there are, however, many Diato- 

 macese more difficult. 



As the nature of these markings is a disputed 

 point, the discussion of which is not adapted for an 

 elementary work, I must refer for further details to 

 the ' Micrographic Dictionary ; 3 it may be remarked, 



