462 Stained Glass. 
for 1812 will be found the description of a micrometer of rock 
crystal, which I presented to the Institute, not only for taking 
the diameters of the sun and moon, but also intended to be used 
in difficult problems in naval tactics and geodosy. Not being 
fully satisfied of the degree of precision which I obtained in the 
measurement of the sun, became desirous of destroying, in the 
two solar images, the aberration oceasioned by the unequal re- 
frangibility of the rays which traverse crystallized substances in 
which we develop the effects of double refraction: for this rea~ 
son I renounced the idea of becoming a candidate for the Pe- 
tersburgh medal. vata 
~ 1 owe it to my colleague Arago, to acknowledge that from 
him I first learnt, that the old pieces of stained glass in our 
churches possessed the astonishing property of transmitting ho- 
mogeneous rays only when they were painted with the oxide of 
copper. These pieces of stained glass are either of a beautiful 
red or bright green; for according as the painting is more or 
less fired (cuite) the oxide assumes these colours ; although they 
are very different; and it was by using glass of a red colour that 
I finally attained the achromatism necessary for the precise 
measurement of the sun’s diameter. 
“The Memoirs of the old Academy of Sciences prove that 
M. Monge was the first to make the observation, and subse- 
quently M. Hassenfratz affirms that he made the same remark — 
on painted glass which was stained of a green colour by the 
oxide of copper. ‘This second remark will prove very useful to 
me, if it produces the same effect with glass painted red by 
~ means of the same oxide; and I beseech Messrs. Vauquelin and 
Thenard to assist me in my new inquiries. It is with a piece’ 
of glass painted red with the oxide of copper, that I obtain at 
this instant two images of the sun perfectly well defined, and 
henceforward I have the means of increasing at pleasure the 
.effect of double refraction. Artists who are employed to cut 
prisms of this substance, in order to procure heliometers for 
astronomers, ought to proceed in the éollowing manner. They 
will select very pure and finely crystallized specimens. The 
cube will not give in the direction of the axis any sign of double 
refraction ; but this cube when cut transversely to the axis will 
give two prisms, which will have attained the maximum of 
double refraction. A second cube similar to the first, but cut 
in an opposite direction, will produce an effect similar to the 
first, with this very remarkable difference, that by putting them 
close together in oppesite directions the effect of the double 
refraction will be doubled: in this way we shall procure, with, 
these two cubes, two new cubes producing a quadruple refrac- 
tion. Thus, by multiplying the cubes, we shall increase at plea- 
sure, 
