Errors in Methods of Measuring Rotary Polarization 9 



At about 567 fjifx. it would seem to be a maximum, because here 

 the variation in the intensity of the extreme wave-length used 

 will be greatest. At some point in the interior of the band it 

 will become zero. In passing from the absorption band the error 

 will not be nearly so marked as it was upon entering. Curves 

 L and //, fig. 2, obtained experimentally, are given to show how 

 the error changes in this particular instance. Their ordinates 

 are differences in the rotations, one small division representing 

 0.005^. Curve L is for the alcoholic fuchsin solution 0.000024. 

 H is the same with a concentration of 0.000012. A cell of water 

 giving 4.5° rotation at the D line was between the poles of the 

 magnet. The angle between the elements in the half-shade sys- 

 tem was Lio"". The intensity of the light source was 14 /x/x. 

 .V I cm. length of the solution placed outside the magnetic field 

 was thrown in and out of the path of the light and the differ- 

 ences in the rotation of the water noted. 



The eft'ects of bleaching must be carefully guarded against. 

 After sunlight has been passed for five minutes through solutions 

 of such substances as fuchsin and cyanin the absorption is greatly 

 diminished. 



Much careful work has been done in determining the optical 

 center of gravity of different light filters^ for various parts of 

 the spectrum, and the calibration is done with substances with no 

 particular selective absorption. These filters let through light 

 differing in wave-length by not less than 34 /a/ia or 40 /*//. Hence 

 a slight difference in the luminosity curves will give, with the 

 half -shade system, a comparatively large error in the rotation. 



\Mien the non-absorbing substance was in the field, a rotating 

 sector was experimented with to equalize the intensity of the 

 light ; but, since this diminishes the amplitude of all waves equally 

 and changes the luminosity curve only in proportion to the 

 Purkinjie effect, the spurious rotations were not eliminated. The 

 insertion into the path of the light, outside the magnetic field, 

 of an optical thickness of the substance equal to that in the field 

 in such a manner as to compensate for the unabsorbed beam, 



^H. Landolt, Das optische D> ehunsfsvertnogeti , p. 387. Chr. Winther, 

 Zeiischr. Phys. Chem., July, 1902. p. 1(31. 



263 



