398 M. H. Becquerel's Experimental Investigations 



In order to measure the rotation of the plane of polarization 

 of light, other methods might be employed, by which the 

 wave-length of the light under measurement would be very 

 accurately ascertained ; but, as I have already remarked in 

 the memoir above quoted, these methods do not admit of the 

 same sensibility as the shaded polariscope when the rotations 

 to be measured are very small. In the present researches I 

 propose to determine, at the same time, the magnetic rotations 

 of various gases with luminous rays having the same wave- 

 length, and to compare amongst themselves the magnetic 

 rotatory powers of the same body with luminous rays having 

 different wave-lengths. It has just been shown that the two 

 studies should be pursued side by side. 



I made use of the light of incandescent lime seen either 

 directly or through coloured screens, which were : — 



1. Rather dark-red glasses, made with cuprous oxide. 



2. Light-yellow glasses. 



3. A bluish green glass, which intercepted nearly all the 

 red rays, and chiefly allowed rays to pass bordering on group 

 E in the solar spectrum. 



4. A liquid screen containing ammoniacal copper nitrate, 

 which gave passage to a pencil of luminous rays, the most 

 intense part of which had an average wave-length included 

 between the lines E and F in the solar spectrum. 



These various luminous sources, seen through a column of 

 0*50 metre of carbon bisulphide, gave the following results : — 



The rotation obtained under the same magnetic conditions 

 for the yellow rays D was taken as unity. The divergencies 

 observable in the results relating to the same coloured light 

 arise from the differences in temperature of the source during 

 the various series, and show the importance attaching to the 

 variations in brilliancy of the incandescent lime. 



These results correspond to the light received after a single 

 passage of the luminous rays through the apparatus ; as is 

 seen, they are rather variable with the luminous intensity. 

 The first series was obtained with a particularly feeble inten- 

 sity; while the other results, which are more concordant, 

 correspond to very nearly the maximum temperature that I 

 was able to impart to the cylinder of lime, a condition that I 

 always sought to realize in the experiments. In order to fix 

 the wave-length of each of the reflected images more precisely, 

 I operated in the following manner : — The large tube was 

 withdrawn from the interior of the coils, and the glasses 

 unscrewed and placed inside the first coil, into which I caused 

 the full current from the pile to pass, by detaching the other 

 coils from the circuit. By this means the magnetic intensity 

 became very considerable. I then arranged the optical system 



