216 Lord Rayleigh : Is Rotatory Polarization 



from ordinary polarimetry, as, for example, in determining 

 the rotation dne to sugar and other active bodies. But the 

 apparatus needs to be specially mounted upon a long stiff 

 board, itself supported upon a point, so that the absolute 

 direction of the light may be reversed without danger of even 

 the slightest relative displacement of the parts. The board 

 swings round in the horizontal plane ; and if its length is 

 directed from east to west, or from west to east, observations 

 taken at noon (in June) correspond pretty accurately to pro- 

 pagation of the light with or against the earth's motion in its 

 orbit. Similar comparisons at 6 o'clock are nearly inde- 

 pendent of the earth's motion. 



In another respect the experiment is peculiar on account of 

 the enormous amount of the rotation to be dealt with. For 

 sodium light the rotation is 22° per millimetre of quartz, so that 

 the whole rotation is 5500°, or more than 15 complete revo- 

 lutions. In the preliminary experiments, with one of the crystals 

 only, sodium light was employed ; but the observations were 

 unsatisfactory, even although the light was resolved into a 

 spectrum. If the flame was well supplied with salt, the 

 extinction of the D-iine by suitable adjustment of the nicol 

 still left the neighbouring region of the spectrum so bright as 

 to prejudice the observation by lessening the sensitiveness of 

 the eye. This effect, which is quite distinct from what is 

 ordinarily called the broadening of the D-lines and can be 

 made still more pronounced by stimulating the flame with 

 oxygen, does not appear to present itself in any other method 

 of observation, and is of interest in connexion with the theory 

 of luminous emission. A very moderate rotation of the nicol 

 revives the D-lines sufficiently to extinguish the neighbouring 

 spectrum, just as the first glimpse of the limb of the sun after 

 a total eclipse extinguishes the corona *. 



When all five quartzes were brought into use it was hopeless 

 to expect good results from a soda-flame. From the fact 

 that the rotation is as \~ 2 we see that there must be 11° 

 difference of rotation for the two D-lines, so that a satisfactory 

 extinction is out of the question. For the observations about 

 to be recorded a so-called vacuum-tube, charged with helium, 

 was employed, the yellow line (situated close to the D-lines) 



* July 6. — A doubt having suggested itself as to whether this effect 

 might not be due to an actual -whitening of the Bunsen flame, such as 

 sometimes occurs rather unexpectedly, the experiment was repeated with 

 a flame of pure hydrogen. The region of the spectrum in the neigh- 

 bourhood of D was even brighter than before. An attempt to produce an 

 analogous effect with lithium was a failure, apparently in consequence of 

 insufficient brightness of the flame. 



