38 
MR. BARLOW’S ACCOUNT OF THE CONSTRUCTION OF 
these screws ; and when every thing is right, the cell is made fast by four other 
screws, to prevent any trifling blow or other slight accident putting the glass 
again out of adjustment. In this state the telescope may be said to be com- 
pleted ; it has of course to be furnished with a finder, proper eyepieces, an ap- 
paratus for illuminating the field, &c., as in the usual cases. 
With respect to inclosing the fluid, the following, after various trials, appears 
to me to be quite effectual. After the best position has been determined prac- 
tically for the checks forming the fluid lens, these with the ring between them 
ground and polished accurately to the same curves, are applied together, and 
taken into an artificial high temperature, exceeding the greatest at which the 
telescope is ever expected to be used. After remaining here with the fluid 
some time, the space between the glasses is completely filled, immediately 
closed, cooled down by evaporation, and removed into a lower temperature : 
by this means a sudden condensation takes place, an external pressure is 
brought on the checks, and a bubble formed inside, which is of course filled 
with the vapour of the fluid ; the excess of the atmospheric pressure beyond 
that of the vapour being afterwards always acting externally to preserve con- 
tact ; the extreme edges are then sealed by the serum of human blood, or, 
which I believe to be equally efficacious, by strong fish glue and some thin 
pliable metal surface : by this process I have every reason to believe the lens 
becomes as durable as any lens of solid glass. 
At all events I have the satisfaction of stating that my first 3-inch telescope 
has now been completed more than fifteen months, and that no change whatever 
lias taken place in its performance, nor the least perceptible alteration either 
in the quantity or quality of the fluid. I must think, therefore, that the ad- 
vantages to be gained by this means of supplying the flint glass are such as to 
entitle the experiments to an impartial examination ; and I cannot doubt, if the 
prejudice against the use of fluids could be removed, that well directed prac- 
tice would soon lead to the construction of the most perfect and powerful 
instruments on this principle, at a comparatively small expense. I am for 
instance convinced, judging from what has been paid for large object glasses, 
that my telescope, telescope stand, and the building for observation, with 
every other requisite convenience, have been constructed for a less sum than 
would be demanded for the object glass only, if one could be produced of the 
