1880. 



MICROSCOPICAL JOURNAL. 



205 



1'33 (the index of water), and it is 

 very improba])le that this will ever 

 be exceeded to any great extent, as 

 1*50 is the ideal maximum of even 

 an oil-immersion. 



The upper curved surface of the 

 lens is silvered, and beneath the 

 lens, a flat silvered plate, ^ of an 

 inch thick, and corresponding in 

 size and position with the upper 

 flattened surface, is balsamed. 



It will be seen that the incident 

 ray is normal to tlie under-surface, 

 impinges on the curved silvered 

 surface, and is thus thrown back on 

 the plane, or under-surface of the 

 lens, whence the more oblique rays, 

 falling beyond the critical angle, 

 are totally reflected, and converge 

 to a focus, giving a numerical an- 

 gle of 1.30=120^' in balsam. 



The object of placing a silvered 

 glass disk beneath the lens is two- 

 fold : in the first place it reflects 

 the less oblique rays which fall 

 within the critical angle, and in the 

 second it tends to diminish the 

 spherical aberration which in this 

 zone miglit otherwise be felt. 



The stop is placed about \ of an 

 inch, or less, below the condenser, 

 and the opening used is of a lens- 

 shaped form, as giving a broad 

 beam without any appreciable 

 spherical aberration in so narrow a 

 zone of light. 



It will be found that this instru- 

 ment will work through any ordin- 

 ary glass slip, gives a brilliant light, 

 ami. having no refracting surface, 

 is necessarily achromatic, whilst 

 the spherical aberration, as previ- 

 ously pointed out, is inconsiderable. 



If used with a dry lens of the 

 highest power on a balsam-mounted 

 object, the light, unable to pass the 

 upper surface of the covering-glass, 

 is thrown back on the object, giv- 

 ing opaque illumination ; on the 

 other hand, with dry objects adher- 

 ins: to the slide, the well-known 



dark-ground illumination can be 

 obtained with any objective I have 

 yet seen. 



o 



The Illumination of Opaque Ob- 

 jects under High Powers. 



BY ALLEN Y. MOORE. 



For some time past, I have used 

 what I believe to be a new method 

 of illuminating opaque objects un- 

 der high powere, which is so cheap 

 and so effective that I herewith 

 give a description of it. 



The object must be mounted dry, 

 on the cover, and the wall of the cell 

 should be narrow, and of transpa- 

 rent cement, such as Canada balsam, 

 dammar, etc. Now, all that is ne- 

 cessary, is to get the light to enter 

 the slide at the "plus angle,'" and 

 this I accomplish by using a Wood- 

 ward prism d^ having immereion 



Fig. 31. 



contact with the slide. Fig. 31 illus- 

 trates this : a represents the slide, 

 h the cover-glass, c the cell-wall 

 of transparent cement, e the front 

 of the objective, and /' and g the 

 paths of two rays of light. 



Firet, following the path of /", it 

 is easily seen that its action is the 

 same as that of Wenham's reflex 

 illuminator — brilliantly illumina- 

 ting objects, transparent or translu- 

 cent, that may be on the slide. 

 For this pui^pose it is, of course, 

 unnecessary to have the cell- wall 

 transparent. Following the ray 

 ^, it is readily seen that it enters 

 the cover and traverses it, bnlliant- 

 ly illuminating any object adherent 



