128 Polarised Dark-ground Illumination. [Sess. 



Some time ago I had the good fortune to become possessed 

 of a very fine polarising prism, which fits into the substage 

 arrangement of my Eoss microscope ; and it occurred to me 

 that, on account of the largeness of the prism, I might to 

 advantage employ polarisation with medium powers — especially 

 as my substage is capable of carrying a condenser along with 

 the polariser. With this in view, I began to experiment with 

 a half-inch objective and open aperture on diatoms, and was 

 rather startled to find that certain forms polarised quite 

 readily, and that those were the same diatoms that appeared 

 most brilliantly iridescent under dark-ground illumination. 

 No sooner did this strike me than I introduced a stop, or spot, 

 into the substage condenser, thereby producing a dark ground, 

 and saw for the first time what I believe to be one of the 

 finest microscopical effects — namely, polarised diatoms upon a 

 black ground. I soon found that pursuing these observations 

 only served to bring out fresh examples of the magnificence of 

 this illumination, and so I have thought that a short paper to 

 the Society upon this subject might be acceptable. 



This polarising — or, more properly speaking, depolarising — 

 property of the Diatomacese is not in itself surprising, when we 

 remember that they are composed of silica — i.e., quartz or 

 flint — which is a positive uniaxial crystal, and consequently 

 depolarises light. Unfortunately, but like very many other 

 things, good results in polarised dark-ground treatment can 

 only be attained by the exercise of some trouble, and the 

 bringing together of certain factors in their most favourable 

 conditions. To begin with : it is necessary to be possessed of 

 a microscope fitted with a substage which is capable of carry- 

 ing both a polariser and a condenser at the same time. The 

 polarising prism should be large, so as to admit a very power- 

 ful beam of light to the condenser above. In order to make 

 the illumination as bright as possible, a good lamp and a large 

 deep bull's-eye condenser should be used, and the light thrown 

 into the substage from the concave side of the mirror. Owing 

 to the maxinmm power of ordinary micro, illumination being 

 a fixed quantity, only a limited number of diatoms can be 

 selected for dark-ground exhibition — namely, those that are 

 large enough to be properly seen under a medium-power 

 objective. Smaller diatoms, requiring higher powers, either 



