3St 0?i the JJfecl'ioiis of LiglU 



man nail, horn, tortoise shell, cornea of n fish, cornea of a cow, 

 cornea of a man. spermaceti, Rupert's drops, gum Arabic, and 

 eaoutcho7ic. 



1. Rock crijstal. The only specimen of this mineral which I 

 could obtain when I made the preceding cx])eriments, was in the 

 form of a double convex lens about -oo'o ^^ ^" n\i:\\ thick. It ex- 

 hibited only segments of the coloured rings ; but thev vvere very 

 large and brilliant, and afforded me the means of making a very 

 interesting experiment with a plate of agate. 



If a beam of common light is incident upon the neutral axis 

 of this crystal, at such an angle that after reflection from its 

 posterior surface it shall emerge in the direction of its oblique 

 depolarizing axis, the light thus polarized by reflection and de- 

 polarized by transmision through the depolarizing axis will reach 

 the eye in the state of white light. If this light is viewed 

 through agate, one of the coloured segments, suppose green, 

 will be distinctly visible ; but if the agate is turned 90' roimd, 

 the gTcen colour will be converted into red., and in general the 

 colour seen in one position of the agate will be complementary 

 to that which is seen in the other position. When the light, 

 how'ever, is brilliant, another verv singular phsenomenon jjresents 

 itself. If the bright image seen through the agate {■^^reen, the 

 nebulous image, in which it is inclosed, will be red ; and when 

 the bright image is red, the nel;ulous image will be green, and 

 iu general the colour of the ii^l)ulous image will be always com- 

 plementary to that of the bright image. If we emjiloy a prism 

 of Iceland spar to examine tlie depolarized light, the colour of 

 the ordinary image is always complementary to that of the ex- 

 traordinary image. We mav therefore consider the preceding 

 result as an cxperimentiwi crucis, which establishes the opinion 

 respecting the structure of the agate, that has been given iu 

 another part of this paper. 



2. Mica. The coloured rings are distinctly visible in mica, 

 both when the light is transmitted perjiendicularly through the 

 plate, and when it is incident in the direction of its oblique de- 

 polarizing axis. The irregular structure of this mineral, how- 

 ever, and the impossibility of procuring laminre with parallel 

 and even surfaces, prevented me from investigating the pheeno- 

 mcnaof its coloured rings. 



3. yjgate. The only plate of agate in which I have observ-ed 

 the coloured rings, is cut in such a direction that it does not 

 polarize the bright image._ It possesses, however, the faculty of 

 depolarization, and therefore must form two bright images, one 

 of which lies immediately above the other. This plate is about 

 — 'v,r of an inch thick, contains no veins^ and exhibits broad 

 segments of coloared rings. 



4. Oriental 



