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. Mains 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 

 Mains, 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 jixed 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. 



