858 SUMMARY OF CURRENT RESEARCHES RELATING TO 



reflected and refracted, but observers have called attention to the 

 effects got by mounting tho objects on slips of " flashed " or " pot " 

 glass, so as to show colours when the object is viewed transparently, 

 and it is obvious that similar effects could be got by the coloured 

 glasses used as screens ; all may be useful, and there is certainly a 

 charm in trying the various devices. 



Before giving the very simple method, which in my hands pro- 

 duces by far the best effects, I would say that the habit of mounting 

 objects in dark cells, either lined with wax or arranged with black 

 paper pasted behind tho slide, cannot be too much deprecated ; if there 

 can be a worse plan, it is putting an object intended to be looked at 

 by reflected light into a cell built up upon white or opal flashed glass, 

 as the glare of light from the polished surface is destructive of all 

 comfort or precision in examination, and no manipulation on my 

 part has been able to obviate the discomfort inseparable from this 

 well-meant but mistaken arrangement. 



"When an object, then, is absolutely opaque, the following method 

 would seem to fulfil all the conditions for its examination. Half a 

 dozen pill-boxes are selected of a size to drop like caps on to the 

 achromatic condenser, which is assumed to be always in position for 

 use. On the outside flat part of these pasteboard caps are gummed 

 rounds cut from various coloured French unglazed papers; these 

 colours preferably shades of green, from emerald to olive, inasmuch 

 as the chitin, which is so preponderant in all insect-preparations, is 

 a shade of red, and the "complementary colour" of red is of course 

 green. This law of " complementaries " or contrasts will be found 

 to aid most agreeably in the display of an object, and to add much to 

 its distinctness. The primary or secondary colours, or even neutral 

 tints, may be used at will, as a colour- battery of this kind would not 

 be costly, and it is evident that little or no trouble would be involved 

 in the substitution of caps, without any disturbance of the side-light ; 

 or by racking the condenser armed with this colour-cap to form a 

 tinted background of the desired shade or brightness, having the 

 advantage of absolute freedom from glare, and forming a contrast to 

 the local colour of the object under examination. 



Sundry analogous devices have been tried, such as cards like 

 object-slips, covered with coloured unglazed paper put behind the 

 object at various distances ; and they have this advantage, that they 

 can be pitched at an angle and so give the effect of a graduated back- 

 ground, the defect being that they are rather apt to tumble out of the 

 position in which they may have been placed. So the coloured cap to 

 condenser has been preferred for continuous use. 



Thus three methods are advocated for illuminating opaque or 

 semi-opaque objects : — (1) For semi-opaque light, on by bull's-eye or 

 silver side-reflector, and through by achromatic condenser or spot- 

 lens when suitable. (2) For opaque objects uncovered, or mounted 

 in transparent glass cells, the Lieberkiihn partly blackened and 

 revolved during use. (3) Colour-caps used on the condenser, and 

 racked up and down ; or coloured cards below the object, illuminated 

 by bull's-eye or side-reflector. 



