EXAMINATION OF OPAQUE OBJECTS. 179 



brasswork of the extremities of the objectives be so bevelled off, 

 as to allow the illuminating cone to have access to the object.' 

 No method of illuminating large opaque objects by lamp-light 

 is more effective, than the reflection of light- from a concave 

 speculum placed near the side of the object (§ 65) ; this not only 

 affords a brilliant light, which may be equably spread over as 

 large a surface as may be required, but may, by the mode in 

 which it is jointed to its supports, be made to throw its rays 

 upon the object at a great variety of angles, without the neces- 

 sity of moving the lamp, whereby the direction in which the 

 best illumination can be gained, is readily ascertained. If a 

 more intense light and a greater concentration be required, than 

 the speculum will afford by reflecting the diverging rays of the 

 lamp, these may be rendered parallel or slightly convergent by 

 the interposition of the bull's-eye condenser, which, for such a 

 purpose, must have its plane side turned towards the lamp. This 

 speculum cannot be so advantageously used by daylight, the 

 ordinary condensing lens being then decidedly preferable. 



94. If the object which it is desired to examine be of small 

 size, and of a shape and character that render it unsuitable to be 

 laid upon the glass stage-plate, or to be turned over so as to bring 

 each side in turn into the most advantageous position, — as is the 

 case, for example, with the capsules of Mosses, the mouths ot 

 which cannot be conveniently brought into view in this mode, — 

 it may be grasped in the stage-forceps (§ 66), which aftbrd great 

 facility for this kind of manipulation ; or, if it be too minute or 

 delicate to be thus held, it may be taken up upon the head of a 

 small pin, by moistening this with saliva or with a little thin 

 gum-water ; and the pin may then be either held in the stage- 

 forceps, or may be run into the cork at its opposite extremity. 

 By careful manipulation, every part of such an object may be 

 brought under view successively, and may be exposed to every 

 variety of illumination. It is in viewing objects supported in 

 this mode, that the utility of the Lieberkiihn (§ 65) is chiefly felt ; 

 for, as the stage-forceps needs to be shifted into difl'erent po- 

 sitions^ so that the object is sometimes raised above and some- 

 times depressed below the level of the stage, in order to present 

 it under a different aspect, the side illumination, whatever be its 

 source, needs to be newly adjusted with each change in the po- 

 sition of the object; whilst the Lieberkiihn adjusts itself, so to 

 speak, when the object is brought into focus. If the mirror be 

 so mounted that it can be turned considerably out of the axis of 

 the microscope, and the aperture of the stage be sufiiciently large, 

 a light of considerable obliquity may be reflected from the Lie- 



' Since the introduction of the Parabolic illuminator, the occasions on which 

 advantageous recourse can be had to the examination of minute objects with high 

 powers by incident light, have become much less numerous; since these objects are for 

 the most part sufficiently transparent to admit of being illuminated by that instrument ; 

 and when they are so, the view of them which it affords is generally much superior to 

 any that can be gained by the method of illumination desciibed above. 



