386 Prof. D. B. Brace on the 



principle of reversal is often lost sight of in connexion with 

 rotary polarization problems. Thus it is true that by a single 

 reflexion the rotation of a ray is neutralized in returning 

 through an active substance and doubled in a magnetic one. 

 If ; however, two reflexions were used, as may sometimes occur, 

 the relative rotation for the different colours would be doubled 

 in active substances and destroyed in a magnetic field. The 

 total rotation of any one ray, however, woidd depend on its 

 azimuth, which changes sign with respect to the normal to 

 the incident and reflected rays, at each reflexion. The 

 resultant rotation would be the sum of the initial and final 

 azimuths of the vibrations on entering and leaving the re- 

 flecting system. White light, returned through an active 

 substance by two mutually perpendicular surfaces, would not 

 show a recomposition, as it does with a single reflexion. 

 Thus, in the arrangement proposed (fig. 1), equal lengths of 

 right- and left-handed substances, e.g. quartz, would give 

 perfect colour neutralization, as well as plane-polarized light, 

 if the double-mirror compensation, described above, were 

 used. As a length of one metre for each cylinder was con- 

 templated, the question of the realization of the normal 

 sensibility of a half- shade system had to be answered. This 

 requires, for its application, a uniform field of sensible area, 

 a condition apparently unrealizable with paths of such great 

 lengths in double-refracting substances. In this case, we 

 should have the well-known spirals resulting from the passage 

 of white light through equal lengths of R- and L-quartz, since 

 the number of reflexions is even in this case and they do 

 not enter. However,, these spirals, at the centre, intersect 

 each other at right angles, and a partial compensation at this 

 point of the field might be realized by means of a crystal 

 of opposite sign placed so as to form a u crossed " system with 

 the quartz cylinder, the cross in the former being tangent to 

 the spiral of the latter for a short distance from the optical axis. 

 It was proposed to use Iceland spar of the proper thickness 

 to give the best result, if this mode of compensation should 

 prove effective. Quartz is positive and the difference in 

 index between its two rays is only about one-twentieth that 

 of spar, which is negative, so that a cylinder of the latter of 

 much less length would be sufficient. As the area available 

 in the field would necessarily be small, the amount of material 

 for cutting could be reduced by using a small cross-section. 

 Samples of both R- and L-quartz crystals of various sizes 

 were secured, mainly from the Hot Springs region of Arkansas. 

 These were cut into prisms (something over a total length 

 of one metre for each kind) approximately parallel to the 



