Io \ On th UNEQUAL 
the effe@t,. from being magnified, becomes more confpicu- 
ous. 
Tue prifmatic apparatus confifts of a fmall prifm- of brafs, 
whofe three angles are equal. Through this prifm, and paral- 
lel with one of its fides, are bored two holes at a fmall diftance 
from each other, equal in fize to the pupil of the eye. The 
fides of the prifm are ground flat, and there are two bits of 
glafs with parallel fides, of the fame dimenfions as the fides of 
the prifm. There are alfo prifms of the fame fize, and with 
the fame angles of different kinds of glafs, and fome crown- 
glafs prifms, with fmaller angles, which, by being applied to 
the large prifm, or to each other, vary the refracting angle at 
pleafure. 
WHEN it is propofed to try the properties of any fluid, one 
of the fmall plates of glafs is applied over the holes on the fide 
of the brafs prifm. A few drops of the fluid are then dropped 
into the hole ; and when it is full, the other plate is laid over 
the holes upon the oppofite fide, and the whole is fecured by 
tying a bit of pack-thread round the ends. One of the glafs 
prifms is now to be applied to the brafs prifm, contiguous with 
one of the parallel plates, the refracting angles of the two prifms 
being placed in oppofite directions, fo as to form a {mall paral- 
lelepiped. 
Noruine farther is neceffary than to apply the eye to the 
hole which contains the fluid, in fuch a way as to obferve 
through it any bright well defined object. The bars of the 
window anfwer the purpofe very well in the day-time, and the 
moon, or a candle in the night. The intention of the two holes 
is for the fake of greater expedition. The properties of two 
fluids may thus be examined and compared at the fame time. 
As the prifmatic portion of fluid and the glafs prifm have 
equal refracting angles, and refrac in oppofition to each other, 
it will eafily be underftood, that if the object feen through the 
: twa 
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