262 Ofi the Affections of Light 



transmitted through a plate of agate cut bv planes perpendicular 

 to the laminae of which it is composed, suffers polarization like 

 one of the pencils formed by double refraction. If the light 

 thus polarized is incident at a particular angle upon any trans- 

 parent body, so that the plane of reflection is perpendicular to 

 the laminpe of the agate, it will experience a total refraction ; if 

 it is transmitted through another plate of agate, having its la- 

 minae at right angles to those of the plate by which the light is 

 polarised, it will suffer total reflection ; and if it is examined by 

 a prism of Iceland crystal turned round in the hand of the ob- 

 server, it will vanish and reappear in every quadrant of its cir- 

 cular motion. 



The pencil of rays to which this remarkable property is com- 

 municated is surrounded by a large mass of nebulous light, which 

 extends about 7° 30' in length, and 1" T in breadth on each 

 side of the bright image*. This nebulous light never vanished 

 with the bright image which it inclosed, but was obviously af- 

 fected with its different changes, increasing in magnitude as the 

 bright image diminished, and diminishing as the bright image 

 regained its lustre. From this circumstance I was led to con- 

 jecture " that the structure of the agate was in a state of ap- 

 proach, to that particular kind of crystallization which affords 

 double images, and that the nebulous light was an imperfect 

 image arisingfrom that imperfection of structure." 



On the supposition that this conjecture was well founded, I 

 imagined, in conformity with the general analogy of all doubly 

 refracting crystals, that the bright image and the nebulous light 

 were produced by two different refractive powers, and I expected 

 to separate the one from the other by forming the agate into a 

 prism with a considerable refracting angle. Every attempt of 

 this kind, however, was fruitless ; no perceptible separation of 

 the images was effected by any of the prisms which I employed, 

 and I was therefore obliged to abandon this mode of investigation. 



Having procured a plate of agate remarkably thin and trans- 

 parent, I admitted a beam of light from the sky into a dark 

 room through a narrow rectangular aperture. When this aper- 

 ture was viewed through the agate, it was surrounded with a 

 very considerable nebulosity ; and by interposing a prism of Ice- 

 laud spar between the agate and the eye, and giving it a motion 

 of rotation, tlie nebulous light became very dense wlien the 

 briglit image vanished, and almost completely disappeared when 



* On eaci) side of the bright image I have observed a condensation of 

 the nebulous light resembling two imperfect iinnjjes of the luminous body. 

 These imperfect images, which increase in number by inclining the agate, 

 aie slightly tinged with the prismatic colours, \^hich evidently belong to 

 that class of ph;«no!ii<iia which have been so ably treated by Dr, fhomas 

 Young, in liislate work on Medical Literature. 



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