186 PREPARATION OF OBJECTS. 



ware troughs are employed, a piece of sheet-cork loaded with lead 

 must be provided, to answer the same purposes. In carrying on 

 dissections in such a trough, it is frequently desirable to concen- 

 trate additional light upon the part which is being operated on, by 

 means of the smaller Condensing Lens (Fig. 69) ; and when a low 

 magnifying power is wanted, it may be supplied either by a single 

 lens mounted after the manner of Ross's Simple Microscope 

 (Fig. 26, b), or by a pair of Spectacles mounted with the Semi- 

 lenses ordinarily used for Stereoscopes.* Portions of the body 

 under dissection, being floated off when detached, may be conve- 

 niently 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 satisfactorily spread out) ; and may be then placed under 

 the Microscope 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 look- 

 ing 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 Grlass Cells to be hereafter described (Figs. 104-107) 

 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 sufficiently surrounded by fluid. For work of this kind 

 no simple instrument is more generally serviceable than Quekett's 

 Dissecting Microscope (Fig. 29) ; but if higher magnifying powers 

 be needed than this will conveniently afford, recourse may be had 

 to Nachet's Binocular Magnifier (Fig. 30), or to an Erector (§§ 64, 

 65) fitted to a Compound Microscope. In this case support may be 

 provided for the hands on either side, by books or blocks of wood 

 piled up to the requisite height ; but in place of flat ' rests, ' it is 

 much more convenient to provide a pair of inclined planes, 

 sloping away from the stage at an angle of about 30° below the 

 horizon, which may be either solid blocks of wood, or made of two 

 boards hinged together. 



135. The instruments used in Microscopic Dissection are for the 

 most part of the same kind as those which are needed in ordinary 

 minute Anatomical research, such as scalpels, scissors, forceps, 

 &c. ; the fine instruments used in Operations upon the Eye, how- 

 ever, will commonly be found most suitable. A pair of delicate 



* The Author can strongly recommend these Spectacles as useful in a 

 great variety of manipulations which are best performed under a low 

 magnifying power, with the conjoint use of both Eyes. — To those whose 

 researches would be specially aided by the conjoint use of both eyes, 

 armed with a somewhat higher power, he would strongly recommend 

 Smith and Beck's 3-inch Achromatic Binocular Magnifier, which is con- 

 structed on the same principle, allowing the object to be brought very 

 near the eyes, without requiring any uncomfortable convergence of 

 their axes. 



