THE AMERICAN MONTHLY. 



[May, 



in different degrees ; sometimes leav- 

 ing the shell smooth but perfect ; 

 sometimes taking with it the outer 

 lamina and leaving only the inner 

 with its delicate punctation. The 

 fact that the thickening is upon the 

 interspaces between dots is additional 

 evidence that the latter are areola, 

 since they allow the light to pass 

 when the thickened walls ai'ound 

 them make a semi-opaque outline 

 approximating the character of the 

 shell in Etipodiscus argus. But 

 among these roughened specimens I 

 have more frequently found the sepa- 

 rate inner lamina, and this, when once 

 caught by the glass, is always the 

 most convincing proof of the scheme 

 of marking of the valve ; for the film 

 is so homogeneous and even, and the 

 dots upon it are diminished to so fine 

 and regular punctae that the eye is 

 never dazzled, confused, or misled 

 in following its delicate pattern. 



The examination of the several 

 species last referred to, under the ver- 

 tical illuminator with high powers, 

 and as opaque objects with the quar- 

 ter-inch objective, is strongly confirm- 

 atory of the interpretation I have given. 

 Reflected light may be made to flash 

 from the surface of all the finer exam- 

 ples of Cos. subtilis as v^ell as from 

 Actinocyclus and Podosira, so as to 

 show a glassy smoothness, with a play 

 of iridescence in the thinnest speci- 

 mens. This is true of both the con- 

 vex and concave surfaces of the valves. 

 No trace of projecting spherules can 

 be seen in such an examination, 

 though the dots of the shells are of 

 such appreciable magnitude that they 

 would easily be visible as protuber- 

 ances if they were solid spherules. 

 Indeed, with the vertical illuminator 

 and a high power, silicious fragments 

 or broken sand-grains may often be 

 seen lying upon the surface of these 

 shells, very much smaller than the 

 areolae, and demonstrating by the ease 

 with which they are seen, that if the 

 dots approached hemispheres in form 

 they also would be perfectly apparent. 

 This, then, is another strong proof 



that these areolae are contained be- 

 tween smooth and parallel laminae. 



If finally, still using the vertical il- 

 luminator and a high power, we re- 

 view the series of valves beginning 

 with the boldest forms of Tricei'atiu7n 

 and Coschiodiscus., and ending with 

 the finest Actinocyclus and Podosira., 

 we find certain appearances consist- 

 ent throughout the whole range of ex- 

 amination. The areolae, when the 

 surface is carefully brought into focus 

 and the cover correction accurately 

 adjusted, is always an opaque white 

 or grey, whilst the surrounding walls 

 or solid part are darker, becoming 

 even black when close to a dark back- 

 ground. The comparison which I 

 have already made to ice upon a pond, 

 when part of it is solid and clear and 

 part of it porous, very aptly describes 

 this appearance. There is no break 

 in the series. From coarsest to finest 

 the only change is that the areolae 

 grow smaller in fact, and generally 

 smaller also in comparison with the 

 solid parts of the shell ; but the light 

 is reflected from the surface in the 

 same way, and the experiment ends 

 with a conviction that the differing 

 methods of examination all lead to 

 the same conclusion. 



In examining diatoms as opaque 

 objects with the middle and low pow- 

 ers, the appearances vary more than 

 they do with the veitical illuminator, 

 because, as the light is necessarily 

 oblique, its variations of direction 

 produce changes of appearance. 

 Parts w^hich look dead-white with 

 the vertical light may appear dark, 

 and the thicker portions of a shell 

 also change color ; but the changes 

 of manipulation of the mirror giv'e so 

 many variable experiments as to end 

 in strong confirmation of the results 

 reached by the other means. In 

 opaque mounts the thinness of the 

 shell is shown better than in any 

 other form of preparation. From the 

 dense Eupodiscus argus we find 

 every degree of diminishing thickness 

 till we come to an Acthiocyclus lying 

 upon the black slide, its flat disc as 



