158 MALUS. 
ber of crystals endowed with double refraction; but 
when a crystal was proposed for examination, there was 
no way of determining whether it could be classed among 
this description of crystals, until after it had been cut 
into a prism, and trial made whether the image of a very 
narrow body, such as the point of a needle, would be 
double, seen through the two inclined surfaces, whether 
artificial or natural. But in 1811 a member of the 
Academy* showed that it was possible to decide such 
questions, without being restricted to the proof, often 
very difficult, of doubling the image. He proved thus 
the existence of double refraction in the thinnest plates of 
mica, which could in no way have been subjected to the 
former mode of examination. Malus generalized the 
results thus obtained by his friend, in a memoir entitled, 
On the Axis of Refraction of Crystals and Organized 
Substances, read to the Academy August 19, 1811. 
LETTER FROM YOUNG TO MALUS. 
On the 22d of March, 1811, Dr. T. Young wrote to 
Malus, in terms of great courtesy, to inform him that the 
* Arago here alludes to his own discovery of the polarized tints 
displayed by any plate of a doubly refracting crystal when interposed 
between the polarizing and the analyzing parts of the apparatus. By 
this means the eye recognizes at once, by the appearance of colour, the 
existence of double refraction in that crystal plate which might be far 
too minute in the deviation of images it would give to be detected by 
the nicest observation; as well as the existence of polarization in any 
light examined by this test. It was thus that Arago detected polari- 
zation in the light of comets, proving that they shine by reflexion. 
The same principle might be applied, to distinguish on inspection a 
small fixed star from an asteroid, and thus probably enable astrono- 
mers rapidly to discover more of those bodies, were it not that all 
known forms of polarizing apparatus necessarily involve so great a 
loss of light, that the method.would probably be inapplicable to such 
faint objects.— Translator. 
