MICROSCOPE. 



347 



be found much superior to any form of knives 

 for cutting through delicate tissues without 

 disturbing them. Swammerdam is said to 

 have made great use of such implements, in 

 his dissections of insect structures, which, 

 from the accounts of them on record, seem 

 almost to have surpassed any which have been 

 since executed. The curved forceps recom- 

 mended by Mr. Slack, we also have used with 

 much advantage. For more minute dissection 

 we can strongly recommend common needles, 

 and cutting implements which may be easily 

 manufactured from them, by grinding down 

 their sides upon a hone. These may either be 

 fixed in wooden handles, or, which is much 

 better, held in little instruments resembling 

 crayon-holders, specially constructed for their 

 reception. The dissector is thus enabled to give 

 to his needle the effect of a long elastic point or 

 of a short stiff one, by simply altering the part 

 at which it is held. In the ivory handles 

 of these holders, also, there is a receptacle for 

 the needles, which makes the whole of this 

 useful little apparatus complete in itself.* 



Many microscopists, especially on the conti- 

 nent, are in the habit of making great use of the 

 compressorium, an instrument in which an ob- 

 ject may be submitted to graduated pressure 

 between two plates of glass, the parallelism of 

 which is perfectly maintained. The results 

 obtained by such compression, however, must 

 be accepted with great caution, and need to be 

 corrected by those views of the object which 

 are gained without that distortion of it to which 

 it is liable in this method. The class of inves- 

 tigations in which the compressorium is most 

 valuable, is that in which such structures as the 

 minute ovum need to be closely scrutinized, 

 without any further change in their shape than 

 may render their contents more distinctly visi- 

 ble. For such purposes we believe that a 

 steady hand and a well-made aquatic box will 

 answer the purpose sufficiently; but to those 

 who prefer relying on mechanical assistance, 

 the compressorium will be a useful instru- 

 ment. 



We shall now proceed to describe some of 



Fig. 166. 



'5- 



the modes of mounting 



and 



arranging simple 



and compound microscopes, which appear to 

 us most convenient. The first that we shall 

 notice is a form which has cheapness and sim- 

 plicity to recommend it, and which, if well 

 made, is capable of an adjustment sufficiently 

 delicate for all ordinary purposes. It is a 

 slight modification of one of Mr. Pritchard's.f 

 A (fig- 166) is the stand, or basis, into which 

 the pillar is screwed. This may be variously 

 constructed, according to the purpose for which 

 it is designed. If bulk be an object, it may be 

 a round disk of lead ; but if weight must be 

 avoided, a thick tablet of mahogany about six 

 inches square, or five inches by seven, (the 

 longest side being in the direction of the 

 movement of the pillar,) is preferable. If great 



* Such needle-holders are sold by Mr. Pritchard, 

 Fleet-street, London. 



f Microscopic Illustrations, 2nd cd. p. 82. 



\ 



v_ 



A 



Elevation of ordinary compound or simple microscope. 



A, base ; B, hollow pillar, with joint at bot- 

 tom; C, triangular rack; D, nut by which it 

 is moved ; E, bar carrying compound body ; F, 

 other end adapted for simple magnifiers ; G, 

 compound body; H, objective; I, spring fork; 

 M, mirror attached to sliding tube, tightened by 

 the nut, N ; S, stage. 



portability be desired, the pillar may be made 

 to screw into the top of the box that holds the 

 apparatus ; but this plan should not be adopted, 

 unless all the smaller fittings be contained in a 

 tray which may be lifted out, in order that 

 there may be no necessity for opening the box 

 when the instrument is in use. The pillar is a 

 thick tube of brass about six inches long, with 

 a large screw at the bottom for being attached 

 to the stand, and a joint above this, for allow- 

 ing it to be inclined at any angle. If this joint 

 be well constructed, the instrument will remain 

 in any position in which it is placed, without 

 any steadying rod or perceptible vibration. 

 Within the tube is a triangular bar, with a rack 

 cut on its posterior edge ; this may be raised 

 or depressed from the top of the tube, by turn- 

 ing the milled head, which carries a pinion 

 working into the rack. Particular attention 



