402 M. H. Becquerel's Experimental Investigations 



tain in the tube gases containing only a few hundredths of 

 impurities, of which, however, an account is kept. Under 

 these conditions it is certain that the glasses do not occasion 

 any sensible phenomena of elliptical polarization which can 

 confuse the measurements. For instance, I placed in the 

 immediate vicinity of the polariscope a small tube of sugar 

 and water, giving a rotation of 55' for yellow light ; and 

 on measuring this rotation by means of the fourth reflected 

 image, it was found to have exactly the same rotation, 55'. 

 The rays had then traversed forty surfaces of glass. 



It must, however, be remarked that when the tube is very 

 hot the circumference of the glasses becomes unequally heated, 

 and they present slight and momentary indications of strain. 

 The glasses can be set so as to counteract the effects pro- 

 duced ; but it is better to conduct the experiments in such a 

 way as to avoid these perturbations, and to reject the obser- 

 vations which there is reason to believe are affected by this 

 cause of error. In order to ascertain the influence of the 

 glasses, the magnetic rotation through the tube full of air 

 was measured ; then the glasses were unscrewed, and the new 

 rotations obtained measured ; the difference between the two 

 gives the action of the glasses. But this difference itself de- 

 pends upon very small numbers ; and it becomes of conse- 

 quence to try, by special measurements, the value of this 

 correction, since it affects all the other measurements. 



The magnetic intensity at the position of the glasses was 

 ascertained either directly or by means of the curve spoken 

 of at page 323. This curve gives the ratio of the mag- 

 netic intensities inside the first coil, and at any distance from 

 it. For the glasses whose centre is at about 0*14 metre 

 from the exterior surface of the coils, the ratio of the mag- 

 netic intensity experienced has been found to equal 0*067 of 

 that inside the coil. 



A heavy piece of glass placed inside the first coil gave for 

 the yellow light a rotation of 106H0; and at the place of the 

 glasses in the tube it gave 7 7, 1. The ratio of the two rota- 

 tions is also 0*067. I adopted this number, and thus, knowing 

 the magnetic rotations of the glasses inside the first coil 

 (p. 399), was able to deduce from them the corrections 

 sought for with the different colours and different images. 



To this correction a second but less important one must be 

 added. It has been seen that both before and after the 

 successive reflections, the luminous rays pass through the 

 mirrors themselves. These are made of St.-Gobain glass, and 

 are 7*9 millim. thick. The magnetic intensity to which they 

 are subjected has been found to equal 0*026 of that existing 



