1887-88.] Polarised Dark-ground Illniniiiation. 



133 



and from accidental light if they are crossed. These spectral 

 colours can indeed be seen without the use of any apparatus, 

 by simply allowing the light to fall upon a prepared slide of 

 this class. I suppose every one who has handled a slide of 

 P. angulatum has been struck with its glittering colours. 



I have constructed a table embracing some of the more 

 familiar diatoms that I have found suitable for this light, and 

 arranged them in terms of the classification I have en- 

 deavoured to explain. 



In dealing formerly with the subject of dark-ground illum- 

 ination of diatoms, I stated that resolution was not thereby 

 assisted ; but with the class of illumination we are at present 

 considering, the resolving power is undoubtedly augmented. 

 It is, consequently, the addition of the polariscope that brings 

 this about. A few months ago, a writer in one of our science 

 journals announced that, by using the analysing prism in its 

 place on the microscope, he could strengthen the resolving or 

 separating power of his objectives. The matter was immedi- 

 ately investigated, and has been most favourably received by 

 one or two eminent London microscopists ; and, along with 

 several of our own members, I have tried this simple means, 

 and found that it was a decided, and therefore a valuable, 

 improvement. I make no doubt that, in polar dark-ground 

 illumination, the increased definition emanates from the same 

 source. 



In using, say, a |-inch or -J-inch objective, the manipulation 



