THE ELEMENTS OF MICROSCOPY. 271 



essence of the whole of the remarks on illumination hitherto made 

 is that whatever method be emplo}'ed, the light must be brought 

 to a focus upon the object.* 



Thus, then, the slide is first placed upon the stage, and some 

 kind of light thrown through it ; a low power is then focussed 

 upon it, and the mirror, condenser (bull's-eye or substage, or 

 both), and lamp so arranged that the whole field is well and 

 evenly illuminated by the rays from the lamp, collected to a focus 

 upon the object. The above remarks, of course, apply only to 

 cases in which the object is to be viewed as a transparent one, by 

 a gleam of light, of a conical form, thrown directly through it, 

 and for most purposes this is amply sufficient. There are some 

 objects, however, mostly extremely transparent ones, whose full 

 beauty or structure can only be brought out by some form of what 

 is called dark-field illumination. With an ordinary observation by 

 direct illumination many rays enter the objective which have not 

 really been affected by the object at all, viz. : — those which pass 

 round it and those which pass through spaces on it, or perfectly 

 clear portions of it. In dark-field, or, as we prefer to call it, 

 oblique illumination, no rays enter the objective except those 

 which really are sent into it by the object itself, its various por- 

 tions acting as mirrors and deflecting the light which has fallen 

 upon them. The absolute truth of the above statement depends, 

 of course, upon the extent of obliquity of the illumination ; the 

 more oblique it is the darker the background. There are various 

 ways of effecting this, the principle in all being the same, viz. — 

 to secure the absence of direct rays entering the objective 

 itself. 



The simplest method, and for low powers as good an one as 

 any, is to swing the mirror just far enough out of the axis of the 

 microscope to bring about the desired result, and by careful 

 arrangement it is not difficult so to arrange the lamp and mirror 

 that the light from both shall be cast upon the object, and that 

 neither shall throw any into the objective. This is best done by 

 placing the lamp on the left-hand side and swinging the mirror 



* The whole subject will be found treated at considerable length in 

 my " Guide to the Science of Photo- Micrography." (London : Kent and Co.) 

 Price IS. 



