Intelligence and Miscellaneous Articles. 237 



were very nearly true ; but by his improved method he has been led 

 to entirely different and quite erroneous values. 

 I am, Gentlemen, 



Your obedient Servant, 



A. R. Clarke. 



ON THE ROTATORY ACTION WHTCH QUARTZ EXERCISES ON THE 

 PLANE OF POLARIZATION OF THE LEAST-REFRANGIBLE RAYS 

 OF THE SPECTRUM. BY M. P. DESAINS. 



In a memoir inserted in the Annates de Chimie et de Physique, 3 e 

 s6r. vol. xxx., De la Provostaye and I investigated the rotatory action 

 which active substances exert on the calorific rays of the visible spec- 

 trum. I have extended these researches to the obscure part of the 

 solar radiation, and beg leave of the Academy to communicate the 

 results which I have obtained. 



At first, working with rays which, in the spectrum I used, occupied 

 a position beyond the extreme red almost corresponding to the yel- 

 low, I observed that the plane of polarization of these rays only ex- 

 perienced a rotation of 19° when they traversed at right angles 

 a plate of quartz capable of imparting a rotation of 52° to the mean 

 red of the same spectrum. 



This first fact obtained, I worked with rays still less refrangible, 

 in the position on the side of the extreme red corresponding to the 

 blue on the other side ; I found heat-rays the plane of polarization of 

 which only underwent a rotation of 8 or 9 degrees under the action 

 of the quartz plate previously defined. Under equal conditions, the 

 rotation of these rays was thus about one-sixteenth that of the ex- 

 treme violet of M. Biot. Their wave-length would thus be four 

 times that of the violet, if it be true that up to these extreme limits 

 we could, as a just approximation, assume that these rotations are 

 inversely proportional to the squares of the wave-lengths. 



To observe conveniently the very feeble rotation whose value I 

 have given, it is not necessary to work with rays isolated in the dark 

 part of the spectrum. If a solar beam is sent through a pretty thick 

 layer of solution of iodine in bisulphide of carbon prepared by Pro- 

 fessor Tyndall's method, rays only are left which quite resemble 

 those on which the observations above described have been made. 



On this new point I have made a considerable number of experi- 

 ments, of which I will describe one series. 



A well-polarized solar ray, which had passed through a layer of 

 iodized bisulphide, was quite extinguished when my analyzer indi- 

 cated 45 degrees. The interposition of the quartz brought about an 

 action on the rheometer ; but this action again disappeared when 

 the analyzer was brought to the division 55, and all observations 

 made in other azimuths agreed in proving that the rotation was in- 

 deed 10 degrees. Here are the observations : — 



