2 Mr Barlow On the Practical Construction 
has been adopted, as well as the principle of calculation givers 
by the same author; but this latter has been reduced to lan- 
guage more intelligible to general readers. 
For the determination of the dispersive power, the instrument 
invented by Dr Brewster, and described by him in his tc Treatise 
on New Philosophical Instruments,” has been selected as the 
most simple, and as possessing every requisite precision for any 
practical application; and for the principles of computation, 
in this case, the formulae, as given by Boscovich, and copied in 
the work last quoted, are those which, after some comparison, 
have been preferred. 
Lastly, For computing the curvatures, we have taken, as 
decidedly superior to any other, the principles so ably illustrated 
by Mr Herschel in the Philosophical Transactions for 1821, 
and have extended his tables, in order to reduce the labour of 
computation to the least possible quantity. 
In every case, also, actual observations and calculations are 
stated in sufficient detail, to render the whole intelligible to every 
one who has any knowledge of the first principles of mathema- 
tics, and who is supposed to be required to construct an object- 
glass of any given focus, from specimens of flint and plate or 
crown glass, with whose properties, in the first instance, he is 
wholly unacquainted. 
2. Instrument for measuring the Angies of the Prisms , and 
for determining the Refractive Indices. 
The first thing requisite is, for the artist to form for himself 
two small prisms of the flint and crown glass he proposes work- 
ing together, reducing them to an angle of about 80° each ; but 
the exact measure of which must be afterwards determined by 
the instrument described below. 
This is shewn in two elevations, Plate I. Figs. 1. and 2. Here 
sss are three screws, which answer as feet to the instrument, and 
which at the same time serve for adjusting it to verticality. 
AB is a tube firmly attached to the centre of the three branches 
forming its base ; T is an interior tube sliding into the former, 
and by means of which the instrument may be turned in any 
position at pleasure. C is a sort of branch fixed to the interior 
tube, to which, again, is screwed the principal circle, graduated 
