nicol's prism. 131 



the refracting surfaces are perfect, and that no light is lost by absorption in the 

 media. 



It is a curious fact, resulting from the polarizing poAver of a pile of glass 

 plates, that the pile is more transparent when held at an obliquity greater than 

 the angle of polarization than it is at that angle ; and that the transparency in- 

 creases with the obliquity. This is owing to the fact that the light which has 

 been polarized by the first few laminae undergoes very little loss by reilcction 

 on increasing the obliquity; but the amount i^olarizcd in those first refractions 

 increases as the obliquity increases, more rapidly than the loss by reflection of 

 the natural light falling on the same surfaces is increased. The intensity of the 

 transmitted beam, therefore, becomes actually greater as the obliquity is greater: 

 a fact which is the reverse of what happens with a single plate. 



A remarkable fact in regard to the condition of lig-ht emitted at great obli- 

 quities from luminous solids or liquids, was discovered by Mr. Arago. When- 

 ever the light of an incandescent body of either of these classes is examined as 

 it proceeds directly from the body and with no great inclination to the luminous 

 surface, it is found to be unpolarized. But when the rays whose obliquity to 

 the surface is very considerable are the subject of examination, they are found 

 to be partially polarized. Tin; inference is, that these rays have been polarized 

 by refmction ; and hence that they must have originated beneath the surface of 

 the luminous body. From the law of equality between the quantities of light 

 simultaneously polarized by refraction and by reflection, it follows that there 

 is a reflection toward the interior of such bodies, of some of the light which 

 they generate. The light of flames and incandescent gases exhibits no such 

 polarization. 



The light of the sun is always unpolarized, whether it be examined at the 

 limb or at the centre of the disk. From tMs observation, Arago was led to 

 consider the luminous envelope of the sun to be gaseous, and not liquid or solid. 

 An incidental corroboration of the ingenious suggestion of the elder Herschel 

 in regard to the constitution of the solar photosphere, is thus derived from 

 optics; and although that hypothesis is by no means universally received, and 

 though there seems recently to have been manifested an increasing disposition 

 among men of science to call it into question, it w^ill be found diflicult to recon- 

 cile the optical properties of the solar light with any supposition which implies 

 that the luminous surface which we see is either liquid or solid. 



In observations upon polarized light, there are some inconveniences attending 

 the use of a mirror, which, when turned in azimuth, obliges the obsex-ver to 

 change his own position; or of a doubly refracting prism or crystal, which pre- 

 sents two images often not sufiiciently separated. Both these disadvantages 

 are obviated by means of a prism invented by Mr. Nicol, which is now in 

 almost universal use. This contrivance is represented in 

 the figure. It is an elongated rhomb, formed of Iceland 

 spar, its length being about three times its breadth. 

 Having been brought into this shape from the natural 

 crystal, it is carefully sawed asunder in the plane which 

 divides it symmetrically through its shortest diagonal, 

 AD, and then reunited by means of Canada balsam. This substance is per- 

 fectly transparent, and has a refracting power whose index is 1.532, interme- 

 diate between those of the ordinary and extraordinary rays, viz., 1.654 and 1.488. 

 The relative index between the crystal and the balsam for the ordinary ra^^ is 

 1.0796, and the limiting angle of emergence from the former to the latter is 68^. 

 The ordinary ray from Pt meets the surfocc, AD, at a greater angle than this, 

 and is totally reflected at O. The extraordinary ray passes through. The 

 sides of the prism are blackened to prevent a second reflection. 



This ingenious contrivance is invaluable to the observer in this interesting 

 branch of optical investigation. Its advantages are, however, in some respects 

 limited. The necessary length of the prism, as compared with its lateral di- 



