1814.] made during the Year 1813. 7 



the same character with one of the pencils formed by doubly re- 

 fracting crystals. 



" 7. This property of light, wliether communicated by the agate, 

 or by double refraction, or by reflection from transparent bodies, 

 may be destroyed by transmitting the h'ght, in one direction, 

 through almost all mineral substances, and even through horn, 

 tortoise 'shell, and gum arabic ; while in another direction the 

 original character of the ray is not altered. The axis of the sub- 

 stance in which the property is destroyed, I have called the depo- 

 Lanzlng axis ; and the axis in which it is not altered, tl>e neutral 

 axis. 



" 8, Mica and topaz, while they possess, in common with other 

 bodies, the neutral and depolarizing axes, have also axes of a diffe- 

 rent kind. Each depolarizing axis of the mica is accompanied 

 with an ollique neutral axis, while the neutral axis, between the 

 two common depolarizing axes, has an ollique depolarizing axis. 



" 9. When the images of a luminous object are depolarized by the 

 mica, they exhibit, by a gentle inclination of the plate, the most 

 singular alternations of the prismatic colours. The same colours 

 were observed in the topaz; and, in a more perfect manner, in a 

 rhomboid of Iceland spar, which exhibited some new phenomena. 



" 10. Light suffers a peculiar modification when reflected from 

 the oxidated surface of polished steel, which seems to prove that 

 tiie oxide is a thin transparent film. 



"11. Light is partially polarized when reflected from polished 

 metallic surfaces. 



" 12. The light reflected from the clouds, the blue light of the 

 sky, and the light which forms the rainbow, are all polarized. 



<' 13. It appears, from a great variety of experiments, that bodies 

 exert a different action upon the different coloured rays, oil of cassia 

 having the least, and sulphuric acid the greatest, action upon green 

 light. 



" 14. The existence of a third, or a tertiary spectrum, lias been 

 established by numerous experiments ; and a method has been 

 pointed out of employing this spectrum as a measure of the action 

 which different bodies exercise upon the differently coloured rays." 



Malus before his death discovered that light obliquely refracted 

 tlirougii transparent bodies is likewise polarized, and this subject 

 has been prosecuted by Arrago. 



Several curious oj*tical instruments have been contrived by Dr. 

 ^^ ollaston and Dr. Young, which deserve to be enumerated amonj; 

 the improvements in Optics. 



Dr. VV%>llaston's pcriscopic camera ohsaira is described in the 

 PhilosophicalTransactions for \M'l. It enlarges the field of dis- 

 tinct vision, and is remarkable for that simplicity which charac- 

 terizes all the inventions of tjjis ingenious philosopher. 



His single lens micrometer is described in the Philosophical 

 Transactions for I SI. i. It is destined to measure the diameter of 

 bmall bodies, which it does with great accuracy and simplicity. An 



