1884.] 



MICROSCOPICAL JOURNAL. 



67 



objective. As a matter of fact, the 

 'eye-sjDots' in the separated inner 

 phite of Coscinodiscus oculus h'idis 

 are so easily seen with a ^-^ objective 

 and a two-inch ocular that I am in 

 the habit of using this glass on the 

 double nose-piece as a ' finder ' when 

 studying that shell in the large variety 

 found in the Nottingham and Calvert 

 County deposits. Therefore, in an 

 opaque preparation of this shell, or 

 of Triceratium favus^ since we are 

 able not only to get an amplification 

 of 400 or 500 diameters by the use of 

 high oculars with the glasses named, 

 but by using a quarter-inch with long 

 working distance may considerably 

 inci'ease the magnifying power, the 

 supposed holes in such shells are far 

 within the limit of common observa- 

 tion by reflected light, and should 

 easily be seen in such slides as Mol- 

 ler's opaque Cuxhaven diatoms which 

 I have already referred to. The truth 

 is, however, that with trifling care in 

 the manipulation of the light the con- 

 tinuous surface of the inner lamina of 

 T. favus may be seen with a clear- 

 ness which defies all skepticism, and 

 if the glass is a good one, there need 

 be no great difficulty in seeing upon 

 its surface the finer system of dots 

 which is independent of the hexag- 

 onal marking, as in the case of Eu- 

 podiscus argtis also. The outer lam- 

 ina will also be found continuous. 

 There is no room for illusion in this 

 matter. Broken shells are easily 

 found, and some with holes broken 

 in them, and the difference between 

 a plane surface and a solution of con- 

 tinuity is too plain to be doubted. 

 The same facts will appear in exam- 

 ining the specimens of Coscinodiscus 

 on the Cuxhaven slide, but as these 

 are much smaller than those which 

 occur in the Maryland deposits I have 

 named, I prefer to use the latter as 

 the tests in regaixl to this species. 



Mr. Peticolas, of Richmond, Va., 

 has mounted slides of the Nottingham 

 earth which happen to contain a larger 

 proportion than common of the ro- 

 bust forms. Upon an ordinary dry 



slide of this sort one may use the ver- 

 tical illuminator and immersion 

 glasses of greatest aperture. Here 

 also we get a view of the diatoms as 

 opaque objects, and I can only repeat 

 that whether the inner or the outer 

 plate of the valve is examined, the 

 closing of the hexagons by a film is 

 as apparent as in examining with the 

 naked eye a real honeycomb which 

 the bees have capped with wax. 



In this investigation we may make 

 a combined use of reflected and trans- 

 mitted light which is instructive. I 

 was led to do this some years ago in 

 examining some slides of diatoins 

 from the sea mud at Savannah, Ga., 

 in which the late Mr. Stodder found 

 what he thought was a new species 

 of Coscinodiscus^ which he provis- 

 ionally called C. compl exus . When 

 photographed the shell showed a pe- 

 culiar lathe-turned marking, made 

 up of mingled dots and triangles. It 

 was, as I thought, essentially the 

 same shell as C. radiatus^ with the 

 walls of the hexagons abnormally 

 thickened, either by an extra deposit 

 of silex or in soine other way. As a 

 transcript made at the time of an ex- 

 amination closely akin to that under 

 consideration, I ma}^ be permitted to 

 quote from a letter I wrote to Mr. 

 Stodder under date of July 30th, 

 1879. After describing one speci- 

 men, a whole shell, I said : ' The sec- 

 ond shell was one which you had 

 marked with the "Maltwood" as 

 having one of the laminag in part 

 broken away. It fortunately turned 

 out, also, to be with the convex side 

 up, and enabled me to make what I 

 must regard as an experimentum 

 crucis. Its broken surface was pe- 

 culiarly adapted to bring out the val- 

 uable qualities of the vertical illumi- 

 nator. I first focussed on the lower 

 lamina, where the upper was entirely 

 removed from it. This was not quite 

 in contact with the cover-glass, and 

 consequentl}' could not be seen so 

 easily as it otherwise would hav^e 

 been. The refraction of the light 

 made it appear black (as a very thin' 



