168 MANAGEMENT OP THE MICROSCOPE. 



more trouble. Another use is made of this adjustment by Messrs. 

 Smith and Beck ; namely, to correct the performance of the ob- 

 jectives, which is disturbed by the increase of distance between 

 the objective and the eye-piece, that is occasioned by the use of 

 the draw-tube (§ 43). Accordingly, they mark a scale of inches 

 on the draw-tube (which is useful for many other purposes), and 

 direct that for every inch the body is lengthened, the screw-collar 

 of the objective shall be moved through a certain number of 

 divisions. 



85. Arrangement for Transparent Objects. — If the object be 

 already " mounted" in a slide, nothing more is necessary, in 

 order to bring it into the right position for viewing it, than to 

 lay the slide upon the object-platform of the stage, and to sup- 

 port it in such a position (by means of the sliding ledge or other 

 contrivance) that the part to be viewed is, as nearly as can be 

 guessed, in the centre of the aperture of the stage, and therefore 

 in a line with the axis of the body. If the object be not 

 " mounted," and be of such a kind that it is best seen dry, it 

 may be simply laid upon the glass stage-plate (§ 67), the ledge of 

 which will prevent it from slipping off when the microscope is 

 inclined, and a plate of thin glass may be laid over it for its pro- 

 tection, if its delicacy should seem to render this desirable. If, 

 again, it be disposed to curl up, so that a slight pressure is 

 needed to flatten or extend it, recourse may be had to the use of 

 the aquatic box (§ 68), or of the compressorium (§ 70), no liquid, 

 however, being introduced between the surfaces of glass. In a 

 very large proportion of cases, however, either the objects to be 

 examined are already floating in fluid, or it is preferable to 

 examine them in fluid, on account of the greater distinctness 

 with which they may be seen ; if such objects be minute, and 

 the quantity of liquid be small, the drop is simply to be laid on 

 a slip of glass, and covered with a plate of thin glass ; if the 

 object or the quantity of liquid be larger, it will be better to 

 place it in the aquatic box; whilst, if the objects have dimen- 

 sions which render even this inconvenient, the zoophyte trough 

 (§ 69) will afibrd the best medium for its examination. If it be 

 wished to have recourse to compression, for the expansion or 

 flattening of the object, this may be made upon the ordinary 

 slide, by pressing down the thin glass cover with a pointed 

 stick ; and this method, which allows the pressure to be applied 

 where it may chance to be most required, will generally be 

 found preferable for delicate, portions of tissue which are easily 

 spread out, and which, in fact, require little other compression 

 than is aflbrded by the weight of the glass cover, and by the 

 capillary attraction which draws it into proximity with the slide 

 beneath. A firmer and more enduring pressure may be exerted 

 by the dexterous management of a well-constructed aquatic 

 box; and this method is peculiarly valuable for confining the 

 movements of minute animals, so as to keep them at rest under 



