6 Fred J. Bates 



In comparing a non-absorbing with a highly absorbing solu- 

 tion the difference in the intensity of the hght reaching the eye 

 is very great. The eye is thus unfitted for rapid setting from 

 the non-absorbing to the absorbing substance, and it is not de- 

 sirable to allow a considerable interval of time to elapse between 

 such observations. In order to overcome this and also to elimi- 

 nate spurious rotations which are discussed in a succeeding paper, 

 an auxiliary cell of the solution with the same absorbing power 

 as the solution tube was placed before the collimating slit and 

 outside the magnetic field. By means of the lever the observer 

 threw thiL cell into the path of the light simultaneously with the 

 tube of solvent. The luminosity curve, as well as the intensity 

 of the light reaching the eye, was therefore the same for both 

 tubes. 



The tubes, one filled with solvent, the other with solution, were 

 placed in the star-shaped frame, the lower portion of which was 

 emersed in the cell filled with the solvent, fig. I. A setting was 

 now made with the solution, say, in the magnetic field; this tube 

 was then thrown out and the tube containing solvent thrown in, 

 and a new setting made. The next settings were from solvent 

 to solution. The time required to exchange the tubes was a 

 fraction of a second. 



The measurements are given in Table I. The numbers oppo- 

 site a wave-length are the differences in thousandths of a degree 

 in the rotations for centimeter lengths of solvent and solution. 

 The minus sign before a number indicates that the rotaticm of 

 the solution was less than that of the solvent. The concentra- 

 tions are parts by weight. 



270 



