Williams^ on the Martin Microscope. 35 
belonging to the instrument, this is merely conjecture. The 
whole height of the instrument, from the table to the top of 
the eye-piece, is a little more than two feet. 
The optical part of the instrument consists of the eye-piece 
and powers. The eye-piece is large, and is composed of three 
lenses, two of which are plano-convex. The middle glass of 
these three, or field glass, is very thick, being almost a bulPs- 
eye. This arrangement ensures a very large field. The dis- 
tance of the two upper glasses from the lower one can be 
increased by means of a rack and pinion, and thus a greater 
amount of power obtained. There are two tubes at the lower 
extremity of the body, into which the powers, &c., are to be 
screwed — an inner and an outer one. The outer can be ad- 
justed by means of a rack and pinion. The use of this ar- 
rangement will appear as we proceed. 
The powers are numerous. According to their marks, 
ten of them range from four inches to -rxjih. of an inch in 
focal length. These are also numbered from 1 to 10, No. 1 
being the highest power. In addition to these, in one of the 
drawers are three higher powers, which appear to have been an 
after- thought. The focal lengths of these are marked respec- 
tively -iVth, -oV th, and ^^^th. There are also four powers, marked 
Yirth, -yVth, 3^^^, and -^t\ and numbered from 1 to 4. 
These last cannot be used with the compound body, but as single 
lenses only, and for their employment a small arm is provided, 
which fits on to the back of the instrument, and can be turned 
round with the head before mentioned, so as to be brought 
into a proper position for use. The V^th is the smallest ground 
lens I have ever seen ; it is scarcely larger than the aperture 
of its setting, which is a mere needle-hole of very small size. 
There are nine Lieberkuhns of various sizes, for viewing 
opaque objects, belonging to this instrument. Six of these 
are provided with lenses suited to their focal length. These 
screw into the inner tube of the eye-piece ; the other three 
are without lenses, and screw into the outer tube of the 
eye-piece, which, being moveable by means of the rack and 
pinion, the observer is thereby enabled to adapt the Lieber- 
kuhn to the power he may be using. 
I have now to describe the apparatus for various purposes 
connected with this instrument; and here I must express 
my regret that so much of it appears to be irrecoverably lost ; 
for, from the numerous vacancies in the drawers and cases, 
it is evident that a number of pieces of apparatus of different 
kinds must be missing, and it is not improbable that many 
of these might have been found among the various lots of 
miscellaneous articles which formed part of the sale, could 
