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V. — Oblique Illumination for the Binocular. 

 By W. K. Bridgman, L.D.S. 



The management of oblique light with the .Monocular instrument 

 is comparatively an easy matter, but when it comes to the bino- 

 cular, it is altogether a very different affair ; in fact, with the 

 ordinary mirror turned out of the axial line as commonly practised, 

 it is scarcely possible to obtain an equal degree of illumination in 

 both tubes simultaneously, and why it should be so will be very 

 apparent on consideration of the conditions produced. When the 

 reflector is turned to either side of the stage, the light falling 

 obliquely upon the surface of glass necessarily becomes graduated 

 in intensity from one side to the other, and as the prism bisects the 

 field in the same line of gradation, it divides the illumination from 

 A to B, Fig. 1, un- 

 equally, by giving the FlG - l - 

 lighter half to one tube 

 and the darker half to 

 the other. It will, hence, 

 be obvious that if the 

 prism could be turned 

 one quarter round and 

 made to cross the aper- 

 ture in the direction 

 from C to D, Fig. 2, it 

 would divide the light fairly, so that each half would be a complete 

 counterpart to the other, and both tubes could thus become similarly 

 lighted up at the same time ; but as this is an impracticability, the 

 only alternative will be to alter the direction of the light itself instead. 

 Oblique illumination can of course be obtained by the prism, the 

 parabola, the spot lens, or the " kettledrum," in any direction ; but 

 these appliances are more adapted for special purposes than available 

 for general use. The want is for some plan of illumination that will 

 serve for all kinds of work and be as little trouble as an ordinary 

 condenser. It is quite a mistake to suppose that oblique light is 

 only of service for bringing out the markings upon diatoms and 

 other tests ; it is in reality the most suitable mode of treatment for 

 every kind of object that can be seen by transmitted light from 

 beneath the stage. 



With direct light thrown up from below and a transparent or 

 translucent substance, it is little better than mere shadow- work ; as 

 the thicker or darker portions then only become visible by stopping 

 out more or less of the direct rays, while the illumined surface, or 

 that part which ought to be seen, is the under-portion presented to the 

 mirror and away from the object-glass. In obtaining a good defining 



