DESCRIPTION OF THE ACHROMATIC MICROSCOPE. 123 



vent the hands from slipping. It will be observed, that 

 the rest here given is perfectly straight ; notwithstand- 

 ing, in some cases, it will be more convenient to have it 

 of a semicircular or curved form. There is an arm, 0, 

 which slides in a socket attached to an universal joint. 

 This arm has a spring ring for carrying the magnifiers, 

 which are inserted into it in the same manner as into 

 the plain end of the arm, shewn in several previous 

 figures. The dissecting trough, #, is placed in the 

 moveable stage in the way described in " Micrographia," 

 page 215. This trough, which contains the object to be 

 operated upon, is capable of being turned about in any 

 direction. When the object is opaque, the condensing 

 lens, m, shewn in figs. 29, 30, must be brought into the 

 position, A, to concentrate the light upon it. 



Single or doublet magnifiers are commonly preferred 

 to the compound microscope for dissecting, because the 

 latter instrument inverts the image of the object. The 

 amplifying powers, which are most generally service- 

 able, are those from an inch-and-a-half focus to the one- 

 tenth of an inch. When higher powers are requisite, 

 the lenses should be made of jewels, as they allow Oi 

 more space between the object and the lens than do 

 glass ones. 



The advantages which I conceive this arrangement 

 possesses over others, are as follow : In the first place, 

 the rest or support, being made of wood, does not con- 

 duct the heat away from the hands so freely as if it were 

 of metal. 2dly. It being situated over the central sup- 



