14 Prof. A. W. Wright on the Polarization 



prism, or a bundle of glass plates set at the polarizing angle, by 

 a diminution of the brightness of the object itself. None of 

 them, however, gave results of any value. In resuming the 

 study of the subject some months later^ the attempt was made 

 to find a combination which should give a large field of view, 

 and which, while absorbing as little light as possible, should in- 

 dicate the presence of even small proportions of polarized light, 

 by sufficient variations of intensity to render it available with 

 the faintest visible illumination. 



A Savart in which the tourmaline was replaced by a Nicol, 

 though possessing almost perfect transparency, was found to 

 give too small a field of view, and bands too faint to render it 

 of any service. Another instrument was constructed on a plan 

 similar to that adopted by Mr. Huggins in observations upon 

 Encke's comet*, by placing a large double-image prism in. the 

 end of a tube 18 inches long, the other end of which had a 

 square aperture a little more than an inch in diameter. The 

 distance was so adjusted that the two images just touched with- 

 out overlapping. This seemed to promise well; and on using 

 it differences of intensity were perceived which indicated polari- 

 zation in a plane passing through the sun. Two defects, how- 

 ever, are inherent to this mode of investigation : — one, that if 

 the field is not of uniform brightness throughout, the brighter 

 side of one image may be juxtaposed to the fainter side of the 

 other, thus giving rise to false conclusions ; another is the un- 

 equal sensibility of different parts of the retina. In consequence 

 of this, the one of the images directly viewed seems always the 

 more obscure, and the true relation of their intensities can only 

 be found by indirect vision, the eye being turned to some point 

 in the median line of the images. Although when used with 

 the observance of the necessary conditions this instrument is 

 capable of giving trustworthy indications, it was soon abandoned 

 for a better. 



Among the polariscopic apparatus belonging to the physical 

 cabinet of Yale College, a quartz plate was found, cut perpendi- 

 cularly to the axis, and exhibiting by polarized light an unusual 

 intensity of colour. It is a made, the body of the plate consist- 

 ing of left-handed quartz, through which passes somewhat ex- 

 centrically a band of right-handed quartz, 6*5 millimetres in 

 breadth. This band is not bounded by sharp lines of division 

 on the sides, but by intermediate strips (b, b in the figures), 

 about 2 millimetres in breadth, which are of different structure, 

 and are apparently formed by the interleaving of the strata of 

 the two portions at their edges. In the polarizing apparatus 

 these strips simply vary from bright to dark, without marked 

 * Phil. Mag. S. 4. vol. xliii. p. 382. 



