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Aiit. XVI. — Description of a Teinoscope^ for altering the 
Lineal Proportions of Objects^ with Observations on Profes- 
sor Am lei's Memoir on Telescopes without Lenses. By David 
Brewstee, LL. D.j F. R. S. Lond., & Sec. R. S. Ed. 
JL HE Instrument which I propose to describe in the follow- 
ing Paper, was invented and constructed in its simplest form 
about the beginning of the year 1812, after I had determined, 
by numerous experiments, that Colourless R fraction may be ob- 
tained by the action of two prisms f the same substance. These 
Experiments, and this conclusion, were published in March 1813, 
in my Treatise on New Philosophical Instruments *[*. 
The only practical purposes to which this singular principle 
seemed to be applicable, were the construction of an achroma- 
tic telescope, with lenses of the same glass, and the construc- 
tion of an instrument for altering the lineal proportions of ob- 
jects. 
The method of imitating the action of the prisms by means 
of lenses, is described in the work already quoted (p. 400.), but 
from the want of a variety of deep meniscuses, I did not ob- 
tain a combination which removed entirely the chromatic aber- 
ration^ although it was quite certain that this could be easily 
accomplished. A little reflection, indeed, convinced me, that it 
was impossible to remove the aberration of figure without mul- 
tiplying the lenses ; — ^that as one kind of aberration was correct- 
ed by the two lenses, another kind was created ; and that I was 
therefore in search of a combination which required to be more 
complex in order to be, of practical utility. 
In the cou^truction of the Teinoscope there were no practical 
difficulties, When the two prisms of crown-glass were put to- 
gether, so ■ as to give refraction without colour, it was obvious, 
that the lineal dimensions of objects were extended or magnified 
in the plane of refraction. I had therefore an instrument which 
magnified any object, such as the elevation of a building, kc. in 
length, while its breadth remained the same, or which altered the 
* From Titvu to extend., and tkottw to see ; an instrument for examining ob- 
jects in a state of extension. 
t Pages 383, 384 ; 399, 400. 
