202 PKEPAEATION OF OBJECTS. 



swer the same puposes. In carrying on dissections in such a 

 trough, it is frequently desirable to concentrate additional light 

 upon the part which is being operated on, by means of the 

 smaller condensing lens (Fig. 45) ; and when magnifying power 

 is wanted, it may be supplied either by a single lens, mounted 

 after the manner of Eoss's Simple Microscope (Fig. 14, b), or by 

 a Compound body mounted as in one of Mr. Warington's ar- 

 rangements (Fig. 24). Portions of the body under dissection, 

 being floated off when detached, may be conveniently taken up 

 from the trough by placing a slip of glass beneath them (which 

 is often the only mode in which delicate membranes can be satis- 

 factorily spread out) ; and may be then placed under the micro- 

 scope for minute examination, being first covered with thin 

 glass, beneath the edges of which is to be introduced a little of 

 the liquid wherein the dissection is being carried on. Where the 

 body under dissection is so transparent, that more advantage is 

 gained by transmitting light through it, than by looking at it as 

 an opaque object, the trough should have a glass bottom ; and 

 for this purpose, unless the body be of unusual size, some of the 

 glass "cells" to be hereafter described (§§ 136, 137) will usually 

 answer very well. The finest dissections may often be best 

 made upon ordinary slips of glass ; care being taken to keep the 

 object sufiiciently surrounded by fluid. For work of this kind, 

 no simple instrument is more generally serviceable than Mr. 

 Quekett's Dissecting Microscope (Fig. 17) ; but if higher magni- 

 fying powers be needed than this will conveniently afford, re- 

 course may be had to Smith and Beck's Dissecting Microscope 

 (Fig. 29), which for this purpose should always be furnished with 

 the Erector (Fig. 32). A particular arrangement of the light, 

 devised many years since by the Author, will enable an expert 

 dissector to prosecute his work with the naked eye, to an extent 

 for which a lens would otherwise be required. This consists in 

 giving to the object the same kind of blaclv-ground illumination, 

 as is now in common use for a very dift'erent purpose; and 

 nothing more is necessary to afford it, than to attach to the under 

 side of the stage a sort of " well," composed of a tube blackened 

 in its interior, about IJ inch long, of the same diameter as the 

 opening of the stage-plate, into the lower extremity of which a 

 diaphragm or a ground-glass may be fitted, for the purpose of 

 diminishing or of softening the light. The slide being laid upon 

 the stage, and the mirror being so turned as to illuminate the 

 object, the eye is to be so placed (the arm carrying the magnifiers 

 being turned to one side) that the object is seen against the dark 

 background afforded by the side of the well. In this manner, 

 fibres of extreme minuteness, or other particles of extraordinary 

 delicacy, can be clearly distinguished, such as could otherwise 

 be scarcely discerned at all without the assistance of a magnifier. 

 And the further the dissection can be carried in this mode, the 

 less difficulty will be found in completing it, when the simple or 



