6o On Dark-ground Illuuiination. [Sess. 



they are calcareous their reflective power is impaired, and no 

 colour is detected in their illumination by this process. The 

 optical difference between diatoms and foraminifera is some- 

 what the same as between ice and snow. Reflection from ice 

 enters the eye from one extended surface at a time ; when this 

 reflection is lost you have transparency : so you have with 

 many diatoms in certain positions of the illuminator. In snow, 

 on the other hand, you have the same iridescent medium; but 

 each particle throws its reflection upon its neighbour, and the 

 appearance of opacity is created. The only theory, therefore, 

 that I can give for the appearance of more than one colour upon 

 certain diatoms, is that they must be possessed of an irregularity 

 of surface sufficient to intercept two or more rays of the spec- 

 tral focus at the same time, and that the minute planes of re- 

 flection upon that surface are not turned towards each other, 

 but reflect the particular ray they impinge upon straight up- 

 wards to the objective, and thence to the eye. 



I have recently examined upwards of 150 different diatoms 

 by dark-ground illumination, under powers ranging from 50 to 

 600 diameters, and in doing so discovered that, when using 

 high powers, a better result is got by placing a drop of water 

 or glycerine between the top of the condenser and the slide. 

 Those of you who are familiar with the theory of immersion 

 objectives, or the principles of refraction generally, will readily 

 understand in what way the benefit arises. Curiously enough, 

 a few days ai'ter I had noted this effect I stumbled upon a 

 short communication to the American ' Micro Journal,' 1884, 

 in which Mr Grunow of New York points out the very same 

 thing. From my cursory examinations I find there are many 

 diatoms unaffected by chromatic light, and appear perfectly 

 transparent. Of these, Navicula rhomboides may be taken 

 as an example. Another class take on from a bluish-white to 

 a deep blue, as, for instance, the Pleurosigmae. Others appear 

 in orange, yellow, or green ; while a select few are radiant in 

 variegated colours. Selecting some of the most conspicuous, 

 I find that Aulacodiscus pruinosus shows its flowery markings 

 in bluish-white ; Gomphonema geminatum in the same colour^ 

 with black dotting very distinct. The spokes and rings of' 

 Arachnoidiscus are made to stand out in relief, the black dots 

 are well seen, and the differences between A. ornatus, A. japon- 



