colours which accompany them in calcareous spar. 289 
the two images overlap, the parts that overlap will be white, 
in consequence of the combination of the two opposite colours. 
The object of using the lens n , is to shorten the tube, but if 
we remove the eye glass, the aperture 0, may be made of any 
size, and placed at any distance from the eye. 
I have been induced to give this particular account of the pre- 
ceding instrument, as another instrument upon the same prin- 
ciple, but of a most unphilosophical construction, has recently 
been exhibited in Edinburgh as a new invention, without any 
mention having been made either of M. Arago or myself, 
who separately discovered the property of polarised light on 
which it depends, or of M. Rochon who invented the eye- 
piece of the instrument. It consists of a tube from twelve to 
twenty inches in length for the purpose of producing a suffi- 
cient separation of the images, and a large object glass of 
rock crystal is placed at the very end of the tube, although 
a small piece -i-th of the size would have answered much 
better, and admitted of a larger aperture if placed near the 
eye. In order to polarise the light, the operator carries a 
large square plate of japanned metal, and places it as near the 
polarising angle as he can. The instrument is then directed 
to this plate, and exhibits two overlapping images affected 
with the complementary colours. 
The investigation in the preceding pages, furnishes us with 
a principle for constructing a new antichromatic instrument , 
far superior to any of the preceding, and so very simple, that 
any person can make it for himself. It is represented in 
Fig. 11, where MNOP is a tube about two inches long at- 
tached to a ball and socket. The end MO of the tube car- 
ries an aperture of any form, and the ball CD contains two 
MDCCCXV. P p 
