ON THE CHOICE OF A MICROSUOPE. 155 
usually consists of two supports, which carry the body into 
which the glasses fit, together with the stage on which the 
oe rests, and the apparatus necessary for illuminating the 
object. 
But it may not be amiss, before giving our reasons for the 
above advice, to name what constitute the recent improvements 
of the modern microscope. They may be not unfitly classed 
under eight heads, the inventions or contrivances having suc- 
cessively come into vogue between the years 1850-60, in about 
the order in which they are placed. They are as follows :— 
Ist. The circular rack movement ; 2nd. The clamping arc ;* 
3rd. The sub-stage ; 4th. The dark-ground illuminator; 5th. 
The double nose-piece; 6th. The double arm to mirror ; 
7th. The separation of the inner and outer lenses of the 
lower object-glasses ; 8th. Wenham’s binocular arrangement. 
The increase of the angular aperture,+ which of late years has 
been so greatly extended throughout all the powers of the mi- 
croscope, and by which so much additional light is gained for 
the clearer resolution of the minute details of objects, immense 
as that improvement is, has not been included in the preced- 
ing list, because the advance in that direction, through Mr. 
Jackson Lister’s able and zealous promotion of that branch 
of optical science, coupled with the exquisite skill of our 
great opticians, has been carried on steadily since the first 
construction of an effective achromatic object-glass in 1824 
to the present time. The principle is the same now as then, 
only applied more largely, so that the angles in the deepest 
lenses have been widened at length to the very last degree 
that they can advantageously receive. Again, considerable 
improvement has been effected in the achromatic condenser 
by Mr. Andrew and Mr. Thomas Ross, and by Mr. Gillett. 
Its application has lately been simplified and enlarged by 
Mr. Thomas Ross, so as to be capable of use with the lower 
powers of the microscope; and its angular aperture has 
been also greatly increased to improve still further the per- 
formance of the deeper powers, while the light that passes 
through it has been modified through Mr. Gillet’s ingenuity 
by a series of revolving steps to an almost infinitesimal 
extent. But the principle is pretty nearly the same as it was 
when M. Dujardin first introduced it. 
The eye-piece also for giving double the usual field, which 
* This contrivance seems to be peculiar to Mr. Ross’s instruments, at: all 
events as to the-manner by which the result is obtained. 
+ For a clear explanation of the term angular aperture, see Mr. Prit- 
chard’s, given in Mr. Quekett’s ‘Treatise on the Microscope’ (p. 426, 
first edition). 
