332 Mr Barlow's account of the Construction of 



of the telescope as usual ; except that the last cell is sufficient- 

 ly large to admit of adjusting the interior one carrying the 

 lens by means of two pair of opposite pushhig screws. These 

 provisions being made, the telescope is placed opposite to a 

 proper object, the centering is produced by trial, by means 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 completed. It has 

 of course to be furnished with a finder, proper eye-pieces, an 

 apparatus for illuminating the field, &c. as in the usual cases. 



With respect to inclosing the fluid, the following, after va- 

 rious trials, appears to me to be quite effectual. After the 

 best position has been determined practically for the checks 

 forming the fluid lens, these with the ring between then[i 

 ground and polished accurately to the same curves are ap- 

 plied 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 cheeks, 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 pre- 

 serve contact ; 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 soHd 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 advantages to be gained by this means of supplying the 

 flint glass are such as to entitle the experiments to an impar- 

 tial examination ; and I cannot doubt, if the prejudice against 



