87 



images being red and the middle green ; at another quarter revolution, 

 four images, but with opposite colours, will be observed, and at the 

 completion of the revolution the original appearance (see fig. 9). 



If instead of the relative positions of the crystals and selenite, as 

 indicated in this experiment, the positions be changed, so that the 

 selenite shall be at 45° from its former position, the ventral axis of the 

 selenite will be parallel or perpendicular to the plane of polarization, 

 and as in that position no depolarization takes place no colours will 

 be produced, and the results will appear as if no selenite were inter- 

 posed. 



The systems of coloured rings, in crystals cut perpendicularly to 

 the principal axis of the crystal, are best seen by screwing the Nicol's 

 prism under the stage and employing the lowest object-glass ; place the 

 crystal over the eye-piece, and use either a short prism or a tourma- 

 line as an analyzer. 



The above remarks are submitted to the notice of the Microscopi- 

 cal Society in the hope of directing the attention of scientific observers 

 to this branch of optical science, in which so much scope still exists 

 for the determination of the laws of various phenomena in natural 

 history. According to the opinion of the eminent author first quoted, 

 " There is scarcely any branch of the subjects of double refraction 

 and polarization which does not afford the richest fields of discovery. 

 Even the theory of undulations, with all its power and all its beauty, 

 is still burdened with difficulties, and cannot claim our implicit 

 assent. It has not yet brought under its dominion the phenomena of 

 elliptic polarization in all its varieties, from the rectilineal polarization 

 of transparent bodies to the almost circular polarization of pure silver. 

 It has not yet explained the singular influence of the force of double 

 refraction over the force which polarizes light, and it has great diffi- 

 culties to struggle with in accounting for certain phenomena of ab- 

 sorption. 



" The determination of the physical data of these departments of 

 science constitutes a new and almost untrodden field, which may be 

 successfully cultivated by almost every variety of talent. The re- 

 fractive indices of the two pencils in all crystallized bodies, measured 

 in reference to fixed points of the spectrum, as has been lately done 

 by Rudberg ; the angles at which light is polarized by reflexion from 

 crystallized and uncrystallized surfaces ; the inclination of the re- 

 sultant axes of crystals having double refraction, for different rays of 

 the spectrum ; the dimensions of the ellipse which regulates the po- 

 larization of metals and their alloys ; the circularly polarizing forces 



