xxvi INTRODUCTION. 



tageous when the most difficult valves of the Diatomaceae, &c. are examined with stops and 

 very high powers and eyepieces, whereby a large amount of light is intercepted. 



Much of the success with which the structure of an object is displayed, will depend upon 

 the manner in which the light is thrown upon or transmitted through it. In general the 

 more hght that can be condensed upon opake objects the better; and when the various 

 parts of such objects are of different colours, the more direct the light and the greater the 

 angular apertm^e of the object-glass, the more clearly will the parts be distinguishable, 

 whilst in certain opake objects which present questionable elevations or depressions on 

 their surface, great obliquity of the incident hght is essential. With transparent objects it 

 is sometimes deshable to diminish the amount of light more or less ; which may be done, 

 either by means of the diaphragm, by using the flat instead of the concave face of the mhror, 

 or by inclining the mirror to one side. It must not be forgotten, in determining the cause 

 of the better display of an object by the substitution of a less amount of oblique light for a 

 larger amount of dhect light, that it need not necessarily arise from the obliquity ; for in 

 many instances the cause is simply the diminution of light, whether direct or oblique being a 

 matter of indifference. When the mirror has only one reflecting surface, the amount of light 

 may be diminished by removing the lamp to a greater distance from the mirror. But the 

 difficulty usually found consists in the amount of light being too small instead of too great. 

 This arises from bad management, and may be overcome by attention to the following circum- 

 stances : the mirror must be placed as near the lamp as possible ; if it cannot be brought 

 within a few inches of the lamp, the shallow bull's-eye condenser must be used : with the 

 object-glasses of high powers the achromatic condenser must be used, and the lower the power 

 of the condensing lenses the greater will be the amount of light transmitted. The lined ap- 

 pearances presented by many objects, require for their exhibition very oblique light, which 

 may be obtained by first raising the mirror as near as possible to the plane of the stage, and 

 then bringing it as much to one side or the other of the stage as can be done : Amici's prism 

 is very useful for producing the same effect in a greater degree ; large angular aperture in the 

 object-glass is also very advantageous under these circumstances, because it will allow of 

 the admission of rays of such degree of obliqiuty as could not enter one of smaller apertm-e. 



In cases where still more light is required than can be obtained in any way by reflexion 

 from the mirror, this must be tm-ned aside, and the direct light of the lamp used. 



Eleven years ago one of the editors* suggested a method of remedying the defects of 

 artificial light, or that ordinarily used to replace daylight. The well-known glare attending 

 lamp or candle light, and the predominance of a yellow colour, so visible when compared 

 with daylight, render it very unfavourable for microscopic pm'poses. It was proposed to 

 mix some substance with the combustible which during its combustion evolved a light of the 

 colom* complementary to (or forming white light with) that predominant in the artificial light, 

 or to pass the light in its passage from the artificial luminary through a piece of glass, of 

 such colour as to intercept or check the objectionable rays. As these rays are of a yellow 

 or reddish-yellow colour, the colom- of the glass must be blue, or purplish blue, but the 

 exact shade must be ascertained by experiment. Thus : the lamp, or whatever som'ce of 

 artificial light it may be, is lighted in the daytime and the light transmitted through the 

 microscope by reflexion in the ordinary way, when its intensely yellowish colour is veiy 

 obvious. Pieces of glass of difierent colours are then separately placed at right angles to the 

 path of the rays from the lamp to the mirror, either close to the flame (in the form of an 

 ordinary lamp glass), upon the face of the mirror itself, beneath the stage, or in an extra 

 head of the side-condenser. If the glass be of the proper tint, and be placed at the proper 

 distance from the light, and in the proper situation, the field will appear as white as the 



* J. W. Griffith, M.D., Med. Gaz Nov. 13, 1843, and Tulk and Henfrey's Anatomical Manipulation, p. 184. 



