233 
determining the dispersive ratio of glass , &c. 
a little towards this object. Probably, also, the immediate 
comparison of the computed results, with experiments on a 
large scale, will add a value to this Paper, which it might 
not otherwise have been thought to possess, and for which I 
am indebted to Messrs. W. and T. Gilbert, who very libe- 
rally engaged to submit to the test of experiments any 
theoretical deductions I might be led to in an investigation of 
these subjects. 
On the determination of the index of refraction. 
2. The following method of determining the index of 
refraction, by means of a lens, is not given as new ; it has, 
on the contrary, been long practised ; but as it forms the 
foundation of the method for determining the dispersive 
ratio, and will occupy but a few lines, I shall be excused for 
introducing it into this Paper. 
It is simply this : — since by knowing the radii of curvature 
of a lens, and its index of refraction, we may compute the 
focal length ; so conversely, by knowing the radii and measur- 
ing the focal length, we may compute the index of refraction. 
The method which we employed for measuring the focal 
length of a lens, was as follows : a tube about 2^ inches in 
diameter, and which exactly measured 10 inches from the 
back of the lens to its other extremity, was fitted with a draw 
tube of the same length, graduated to inches and tenths, and 
which, by means of a vernier, might be read to the hundredth 
of an inch. This was fitted with a positive eye-piece, which 
was adjustable to bring the cross wires exactly into its focus, 
and the graduations above-named commenced from this 
MDCccxxvn. H h 
