52 



PRACTICAL TREATISE ON THE MICROSCOPE. 



The lenses are set in brass frames, which easily fall into the 

 cell, as seen in section in fig. 29, where e represents the lens, 

 and d the frame in which it is set ; and when it is required to 

 change the power, we have merely to turn the cell upside 

 down, the lens wiU drop out and another can be substituted. 

 It may be as well here to state, that the form of the low 

 power lenses employed for the purpose of dissecting should be 

 double-convex, a planoconvex, with its convex side towards 

 the eye, gives a flat field, perfect in the centre, but not 

 at the margins. This form of microscope is exceedingly 

 useful for minute dissections of nerves that are carried on 

 under water in troughs or other vessels, and will be found 

 sufficiently steady for the purpose, the length of the arm 

 allowing the lens to be brought over any part of the trough 

 or vessel in which the dissection is contained, so that the size 

 of the subject to be examined need not be considered. 



When a much more steady instrument is required for the 

 purposes above described, Messrs. Powell and Lealand have 

 contrived a form represented bv fig. 31 ; it consists of a 



brass foot, or base, h, 

 about five inches in 

 diameter, and an inch 

 and a half thick ; to 

 make it more steady, 

 it may be loaded 

 with lead ; from this 

 foot rises a trian- 

 gular stem, a, about 

 twelve inches in 

 length, having a 

 rack, d, on one of 

 its sides ; upon this 

 stem, a square box, 

 c, carrying a pinion 

 and two milled heads, 

 is made to move up 

 and down by the 

 rack. To the box is attached a strong tubular, but conical. 



Fig. 31. 



