1 887-88.] Polarised Dark-ground Illinnination. 1 29 



have their radiancy killed by the inevitable loss of light, or 

 cannot be made to stand on dark-ground by reason of the 

 increased aperture of the objective. For this work I would 

 therefore recommend the use of a half-inch objective of 

 moderate aperture, so that all the frustules in the field of the 

 instrument may be sharp to the edges. Deep eyepiecing con- 

 sumes light, and should be avoided, although I admit that 

 the effect of occasionally inserting the Kelner orthoscopic C is 

 very striking, on account of its enormous field. Lastly, the 

 selenites should be in the form of discs, set on the top of the 

 polariser, and beneath the condenser, so as not to interfere 

 with the focus of the emergent rays from the condenser. 



Considering the somewhat complicated nature of the sub- 

 stage apparatus, at first sight it must seem a matter of diffi- 

 culty to adjust this kind of illumination, but in reality it is 

 not so. The operation is as follows : Transmit the light in 

 the ordinary way, focus the object, and centre the stop of the 

 substage condenser so as to get a perfectly black ground, at 

 the same time being careful not to cross the prisms, otherwise, 

 in the first instance, you will get too much darkness. In con- 

 junction with the adjustment of the stop, it is necessary to 

 rack the condenser up or down until a position is found where 

 the field and object are in greatest contrast — namely, where 

 the object is most brightly lit up and the field darkest. One 

 or two movements of the mirror generally require to be made 

 during this operation of centering. By rotating the analysing 

 prism the splendid effects are then seen, the thickness of the 

 selenite governing the prevailing colours. I wish again to 

 mention that, in order to get bright images, and a large num- 

 ber of them in the field, a low ocular should be used. The 

 brightness may be greatly enhanced by the application of 

 glycerine between the condenser and slide ; and if a slip 

 selenite is used, it also should be brought into contact with 

 the slide by the same means. With your indulgence I will 

 endeavour to describe the effects of polar dark-ground upon a 

 few familiar diatoms, the power used being a half-inch, with 

 an A eyepiece on a ten-inch tube — that is to say, under a 

 magnification of 100 diameters. 



A very slight study will suffice to convince the observer 

 that there are two distinct classes of diatoms — first, iridescent 



