132 POLARISCOPES. 



mensions, renders it difficult to employ light of any considerable convergency 

 or divergency. The cost of the construction of such prisms increases also 

 very rapidly with their magnitude; and few have been made at all which mea- 

 sured more than an inch on the side. Those commonly found with opticians 

 are much smaller than this. 



The eye-piece of Mr. Delezenne is a very good substitute for Nicol's prism; 

 although it affords a less intensit}^ of light. In this, the surface CD is a pol- 

 ished mirror of black glass ; ABD is a prism of trans- 

 parent glass. B.r, Rr, rays of light falling at the pola- 

 rizing incidence upon CD, are reflected at a perpen- 

 dicular incidence upon the first surface of the glass 

 prism ; and after being totally reflected on AB, emerge 

 at right angles to the surface AD, meeting the eye of 

 Fig 15- the observer at 0. 



Another convenient eye-piece, which may also serve, like Nicol's prism, as a 

 polarizer for small beams, is formed of a lamina of tourmaline cut parallel to 

 the axis. This mineral possesses the very remarkable property, when not in 

 exceedingly thin laminre, of suppressing one of the rays into which incident 

 common light is divided by it, and transmitting the other. The ray transmitted, 

 as in Nicol's prison, is the extraordinary ray. Cut perpendicularly to the axis 

 a plate of tourmaline is opaque. Two equal plates, cut parallel to the axis, are 

 opaque when crossed upon each other. 



The disadvantages of the tourmaline eye-piece are, first, the color of the 

 crystal, Avhich mars the beauty of the tints exhibited by polarized light, and to 

 some extent neutralizes them. It is rather unfortunate that the crystals which 

 are least colored are usually bad polarizers. In this respect different crystals 

 very much differ. Some, which are light green, transmit a notable amount of 

 the ordinary ray even when quite thick. Those vv^hich polarize best are usually 

 brown or yellowish broAvn. Occasionally one of this kind will be found which 

 polarizes well without being very disagreeably dark. But an equal if not 

 greater disadvantage of the toiirmaline is the great b'rittleness of the crystal 

 and the rarity of specimens in which fissures do iTOt naturally exist. It is 

 difficult, therefore, to obtain clear plates of any considerable size. Finally, the 

 supply seems, of late years, not to have kept pace with the demand; and op- 

 ticians intimate that it is almost impossible to obtain specimens fit for optical 

 purposes at all. 



A few years since Dr. Herapath, of London, announced the discovery of a 

 property like that of the tourmaline, in artificially prepared crystals of the iodo- 

 disulphate of quinine. These crystals are but slightly colored ; and could they 

 easily be prepared and made permanent would probably come into general use. 

 Dr. Herapath succeeded in obtaining specimens half an inch across. 



The peculiar property of the tourmaline was also early observed by Sir 

 David Brewster, in agate; but that substance is not sufficiently transparent for 

 the ]nirposes of optical experiment. 



When large polarizers are needed, resort must be had to reflection from mir- 

 rors made of black glass, which reflect only from the first surface, or of trans- 

 parent glass whose surfiices are truly parallel. If great purity in the polarized 

 beam is not an object of importance, bundles of thin plates may be employed 

 as heretofore described, to polarize either by reflection or by refraction. 



In the year 1811 Mr. Arago communicated to the Academy of Sciences of 

 Paris, one of the most remarkable and beautiful discoveries which has ever 

 been made in the history of optics. Upon examining thin plates of certain 

 transjiarent crystals, such as mica, selenite, or quartz, by means of transmitted 

 polarized light, he found that when the light was received upon the eye through 

 a prism formed of Iceland spar, the richest^ conceivable colors made their ap- 

 pearance, which were complementary to each other in the two images, and 

 Avhich varied in intensity with the azimuth of the laminse or of the prism. 



