3B6 
Dr Brewster on a Teinoscope 
and to produce within moderate limits any change that was re- 
quired in the ratio of its lineal dimensions. 
In constructing these instruments for actual use, it is not 
necessary that the prism C should be made of the same kind of 
glass as AB. The power of the instrument would be greatly 
increased, by making C of fluor-spar or rock-crystal, or the 
colourless topaz of New Holland on account of their low dis- 
persive power. The reflection of light at the cylindrical sur- 
faces of AB, may be removed by introducing a film of oil of the 
same refractive power as the glass, and the motions of the 
prisms N and C may be so connected, that the achromatic posi- 
tion for different angles of the variable prism may be obtained 
by turning a single screw nut. 
Upon explaining the principles and application of the Teino- 
scope to an eminent artist, he seemed to consider it as too re- 
condite for ordinary use, and I therefore did not publish any 
account of it along with the experiments on which it was found- 
ed. I am persuaded, however, that it will be found of great 
utility in painting, sculpture, architecture, and, in short, every 
branch of art, where it is requisite either to discover just pro- 
jTortions, or to copy those which nature has already displayed. 
I have been induced to print the preceding account of the 
Teinoscope, in consequence of a paper which has been recently 
published by Professor Amici of Modena, in the nineteenth 
volume of the Memorie della Societe Italiana^ entitled Me.- 
moir on the Construction of an Achromatic Telescope without 
lenses^ and of a single Refracting Medium This instru- 
ment is nothing more than the combination of prisms, with this 
difference only, that in my instrument the magnifying power 
may be varied at pleasure, whereas it remains always the same 
in Professor Amici’s. It never occurred to me to call such an 
instrument a telescope, or to propose it for an opera.-glass, as 
has been done by Professor Amici ; and, in like manner, it dees 
• When quartz or topaz are used, the mean position of the refracted ray in 
the prism C, should coincide with the axis of the former, or one of the resultant 
axes of the latter. 
*}■ I have not seen the original Memoir, and am acquainted with its contents 
solely through a short notice in the Quarterly Journal, No. xxiv. p. 400. ; and a 
/uller dialysis of it in the Bihlioiheque Univcrselle, Nov. 1821, p. 1 74. — 184. 
