132 Polarised Dark-ground Illumination. [Sess. 



medium ; but on carefully examining different mounts of the 

 same genera, I have invariably found the dry, and also the 

 chloride of zinc, to give the greatest brilliancy : monobromide 

 does not appear to be suitable for this purpose. With regard 

 to the diatoms I have referred to, and classed as opaque, on 

 which there is no dispersion of light, I am inclined to believe 

 that, while they are in all other conditions the same as their 

 brethren, they present a calcareous appearance, their surface 

 being dead, like that of obscure glass. 



Now, concerning the polarising property of certain diatoms 

 treated with open aperture, I desire particularly to point out 

 that those frustules only polarise which come under the head 

 of my first class — namely, the iridescent ones. Take, as an 

 example, P. angulatum polarised under a blue and yellow 

 selenite. All the valves that appear erect in the field will 

 be coloured, say red, and all those at right angles green. At 

 first sight it seems strange that, by using a blue and yellow 

 selenite, while the field conforms to these colours, the frus- 

 tules should become red and green ; but this arises from the 

 fact that the diatoms have the power of retarding the light 

 one quarter of a wave length, in the same way as a quarter 

 plate of mica. This makes a very pretty as well as instruc- 

 tive subject, and is only one of the many interesting phases 

 under which this diatom shows itself. By removing the sel- 

 enite and crossing the prisms, the highly polar nature of this 

 Pleurosigma becomes more apparent. The valves shine out 

 clearly and distinctly on the dark ground — which, as I re- 

 marked before, proves unmistakably their property of polarisa- 

 tion. Beyond this, however, it will be noticed that each valve 

 retains a slight colour — either a tinge of bluish green, or one 

 here and there from a dark purple to a dingy red. By this I 

 conclude that a minute quantity of light is admitted, on ac- 

 count of the imperfections of the prisms or their centering, 

 which reaches the diatoms in a retarded form, and is partially 

 polarised, partially dispersed; but I am inclined to believe 

 that, while both actions are traceable, dispersion is the prin- 

 cipal colour-producer. If the selenite is now replaced, and 

 the dark ground maintained by the insertion of the stop, 

 both the iridescent and polar colours will be visible — the 

 former directly from interference if the prisms are open, 



