Double Refraction of Quartz. 201 



To use artificial light with facility, a lamp is placed in the 

 focus of a lens whose diameter is equal to the diameter of the 

 circle described by the point of the rhomb. The board which 

 carries the lamp and lens is cut at such an angle that on apply- 

 ing- its sloped end to the side of the board carrying the other 

 apparatus, the light is reflected by the polarizing plate in the 

 proper direction without any farther adjustment. 



The advantages of this apparatus are, that with a very small 

 specimen, by day or by night, it will exhibit all the phenomena 

 of plane and elliptically polarized light to the greatest angular 

 extent that the eye can receive, and with conveniences of mea- 

 surement that have never before been given : that thus a macled 

 crystal, or a piece of unannealed glass, &c. can be as it were 

 dissected by successive examination of different small portions: 

 that any accidental roughness or inequality of the crystal pro- 

 duces no sensible effect: and that by placing the specimen in 

 another position, the macled structure can be exhibited with 

 singular clearness. 



In the examination of the phaenomena which quartz presents 

 when exposed to circularly polarized light, detailed in my paper 

 before alluded to, I found very great difficulty in consequence 

 of the contraction of the field of view when Fresnel's rhomb is 

 used with the common polarizing apparatus. In examining the 

 two spirals inwrapping each other, I could see little more than 

 a single line at a time: and it was only by carefully turning the 

 crystal that I could discover the relation of one line to another. 

 It was in fact principally to overcome this difficulty that I devised 

 the apparatus here described : with this I can at once see the 

 folds of the spirals as far as the colours are sensible. I have 

 much satisfaction in seeing that my delineation was quite correct. 



Vol. IV. Part I. C c 



