56 PRACTICAL TEEATISE ON THE MICROSCOPE. 



lenses of one inch and one half-inch focal length for the dis- 

 section and examination of opaque objects ; but by means of a 

 dissecting table or platform, with a mirror underneath, as 

 described with Mr. Powell's instrument, page 53, it wiU 

 answer equally weU for transparent objects, especially if the 

 dissecting rests, subsequently to be described, be used at the 

 same time ; the joint at f allows of the lens being adjusted 

 with very great nicety. 



This apparatus is also readily taken to pieces, by unscrew- 

 ing the pillar, d, from the stand, and, with the lenses, dis- 

 secting instrmnents, and forceps, is packed in a small case, 

 which can be carried in the pocket. 



These little instruments the Author has found extremely 

 useful for the examination and selection from sand of many of 

 the smaller kinds of foraminiferous shells. A small quantity 

 of the sand supposed to contain them, may be spread on 

 a piece of black paper on the table, and by means of this 

 simple microscope, and a sable or other pencil brush capable 

 of being brought to a fine point, a great deal of work 

 may be performed in a short space of time, and with 

 much more ease than with a compound instrument, in 

 which all the objects are reversed; and as the cost of 

 these microscopes is comparatively trifling, and the uses 

 to which they are applicable so extremely various and 

 important, no student of natural history should be without 

 one. 



The instrument best suited for dissection is one which was 

 described in the forty-ninth volume of the Transactions of the 

 Society of Arts, by Mr. Slack. It consists of a box or case, 

 seven inches high and four inches broad, represented open in 

 fig. 35. The upper siurfaces, r r, are sloped ofi" to four inches 

 square to form arm rests, and the top is left six inches by 

 four. The front of the case is provided with a flap or door, 

 having hinges at the bottom and a lock at the top; the 

 mirror is situated in the bottom of the case, and is of large 

 size, and directly over it, in the top, is an opening, g, an inch 

 and a quarter in diameter, which may be closed, if required, 

 by a brass cap. 



