xxx INTRODUCTION. 



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

 it is sometimes desirable 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 

 mirror, 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 direct 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, or turning this somewhat on one side. But the difficulty usually 

 found consists in the amount of light being too small instead of too great. This may be 

 overcome by attention to the following circumstances : 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 mustjbe made to condense the light upon the mirror : 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 

 appearances 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. 

 Nachet's, Amici's, or Reade'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 a degree of obliquity 

 as could not enter one of smaller aperture. 



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

 reflection from the mirror, this must be turned aside, and the direct light of the lamp 

 used, thus : clamp the slide so that the object projects beyond that side of the stage which 

 is nearest the lamp. The body of the microscope is then rotated so that the object-glass is 

 over the object and fixed by the milled head; the axis of the body being then directed to 

 the light, the object may be thus brought into focus ; and by moving the lamp around the 

 microscope, light of any obliquity may be made to pass through the object. This is a 

 simple way of obtaining the most oblique light, and as the light comes directly from the 

 lamp, there is no loss from reflection, as in the use of prisms. By a little variation of this 

 arrangement, the light maybe made to fall very obliquely upon opaque objects, especially 

 if uncovered. In many instances the use of the direct light of a lamp is highly advan- 

 tageous, and may be applied even when the highest powers are used. It has an advantage 

 over that of the mirror, inasmuch as when the latter is used, the light entering the object 

 is derived from two sources, viz. reflection from the outer and the inner surface of the 

 mirror ; whereby two images are formed, confusing each other ; while with the direct light, 

 the image is single and the definition finer. 



Many years ago I suggested a method of remedying the defects of artificial fight, 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 purposes. It was proposed to 

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

 the colour complementary to (or forming white light with) that predominant in the arti- 

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



