io ; On the UNEQUAL 



the effect, from being magnified, becomes more confpicu- 

 ous. 



The prifmatic apparatus confifts of a fmall prifm of brafs, 

 whofe three angles are equal. Throvigh 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 fmall paral- 

 lelepiped. 



Nothing 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 refract in oppofition to each other, 

 it will eafily be underftood, that if the object feen through the 



two 



