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the submerged tube keeps the water through which the rays 
of light penetrate to the lenses of the microscope as smooth 
and unruffled as itself. Again, the proposed arrangement 
allows us to use object-glasses of much greater power than 
can be employed in the “ tank microscope.” ‘Thus, I find 
that I can work beneath the water perfectly with a power of 
1 inch, and a higher power might be used, only I do not 
think there is much occasion for a higher or even so high a 
power in the investigations likely to be undertaken with this 
instrument. 
I said that I had applied these submersion tubes to the 
object-pieces of an ordinary microscope. ‘The microscope is 
one of Bryson’s, in which the vertical motion is communi- 
cated to the microscope-tube by means of a rack and pinion 
on the tube itself, and not on the supporting pillar, as is the 
case in many microscopes. I place my tank on the movable 
stage of the microscope, which must, of course, be maintained 
in a horizontal position, if by a clamp so much the better. 
The tank is formed by pieces of glass cemented together 
by marine glue. The back and outward-sloping sides 
are made of opaque white glass, the front and bottom of 
clear plate glass. By this arrangement a great amount of 
light is thrown in upon the water, which receives light also 
from the mirror beneath the stage. If it is desired to have 
the object illuminated from above only, a piece of black cloth 
may be placed beneath the tank, or a piece of tinfoil may be 
used for certain cases. 
I have here described the adaptation of submersion tubes 
to an ordinary microscope, but were it thought desirable to 
construct a microscope solely for subaqueous investigations 
the arrangement would, of course, be different to that of ordi- 
nary microscopes. The tank I would make rather larger than 
that at present used by me, the size of which was limited 
by the dimensions of the microscope and its parts,’ or, 
rather, I would have several tanks of various sizes, suited 
for different purposes. The tank should stand on a fixed 
stage, fitted to the upright of the microscope. ‘The micro- 
scope should be made capable of movement in every direc- 
tion, not merely vertically, but horizontally and obliquely, 
and the water in the tank should be capable of being illu- 
minated from the top, the front, or the bottom, as desired. 
I would also extend the length of the object-piece covered 
by its submersion tube, so as to admit of submersion to a 
depth of three or four inches. I had, in fact, designed a 
1 The actual size of my tank is :—depth 23 inches, sides 34 inches wide, 
front 4 inches, back 3 inches wide. 
