OF FLUID REFRACTING TELESCOPES. 
15 
between the two, the usual thickness of plate-glass, as at present manufactured, 
would be sufficient, and we might have the selection from large stores of the 
best glass at a trifling expense ; and as to that of the correcting fluid, or sub- 
stitute for the flint-glass, it is so very inconsiderable as not to deserve being 
mentioned ; although, if it were possible to obtain a piece of flint-glass large 
enough for such a purpose, scarcely any price, however great, would be thought 
exorbitant. In the instrument proposed, nearly the whole expense would be 
the workmanship, and I must think it very inconsiderable in comparison with 
the magnitude and importance of the undertaking. 
I had intended to have concluded this paper by giving the curvature, foci, 
&c. which I have computed ; but as they are merely supposititious, as far as 
they are dependent on the index and dispersion of the front glass, it is perhaps 
better to withhold them. The only object I had in making them was to form 
some idea of the requisite curvature, thickness of glass, See. They can only of 
course be permanently made after the plate-glass has been selected. 
