156 ON THE CHOICE OF A MICROSCOPE. 
has been long in use, has lately been achromatised by Mr. 
Kelner making it a more efficient eye-glass for the popular 
exhibition of coarse objects, such as micrographs, insects 
entire, and transparent sections of wood and stone. The 
same may be said of the traversing motions of the stage 
which have received a range of late years by some opticians 
to the extent of 1+ inches in rectangular directions, but the 
movements are on the same plan as those in use many years 
ago. And so likewise with the selenite stage, which has been 
improved by the late Mr. Darker; as also with Dujardin’s 
prism for reflecting oblique light on transparent objects, 
which has been modified in various ways by M. Nachet and 
Amici. I think, then, the recent improvements of the modern 
microscope may not unreasonably be restricted to the eight 
ones named above, and the several objects of which are, in 
short, as follows: 
lst. The circular rack, which is immediately beneath the 
object stage and is capable of carrying it round in Mr. Powell’s 
instrument the entire revolution of a circle, and in Mr. Ross’ 
3 of a revolution, is a very convenient movement for altering 
the angle at which an object is being viewed without putting 
it out of field or focus, and that even under the deepest 
powers. This circular rack, moreover, being graduated, can 
be used as a goneometer for measuring crystals ; and it may 
be altogether considered as the crowning perfection of the 
rectangular stage motion. 
2nd. The clamping arc is a simple but effective contrivance 
of Mr. Ross’s, by which the microscope can be firmly fixed at 
any inclination, and is a point of consequence after the 
instrument has been long in use, and the suspension joint has 
become too supple at that angle of inclination to which it is 
commonly adjusted. 
3rd. The advantage of the sub-stage “for holding and 
adjusting by universal motions all the illuminating and 
polarising apparatus placed beneath the object” can scarcely 
be overrated, its applications are so various and convenient. 
This sub-stage, besides, can be instantly racked off and 
detached from the instrument when it is wanted to illuminate 
opaque objects with themirror and Lieberkuhn; nothing isthen 
left to intercept the light between the mirror and the objects, 
avery large space being given within the object stage through 
which an extremely oblique light can be thrown upon them. 
4th. The dark ground illuminator, whether by, means of the 
spotted lens for the lower, or of the paraboloid for the higher 
powers, is an admirable contrivance, by which a brilliant light 
is thrown to appearance on semi-opaque objects, though really 
