99 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 cun'es, 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 efiicacious, 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 

 has 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 



