168 Prof. D. B. Brace on a 



e. a. the dispersion may be obtained by a single optical system 

 and the resulting spectrum formed in the plane of the slit above- 

 referred to as conjugate to the analyser. A suitable mirror 

 before the prismatic system may be made to rotate any colour 

 into this slit and maintain the system of rays used homocentric 

 to any axis in the system. It is not easy, however, with such 

 a system to obtain a uniform intensity over the entire field of 

 view. The difficulty from diffusion referred to, however, is 

 eliminated. 



The system which has given thus far the best results is a 

 modification of one devised by the writer for colour mixtures 

 and used by Doubt * in his determination of the colour 

 equations for different radiants. This is a reversing colour- 

 system consisting in reality of a double dispersing system. 

 A concave mirror whose radius of curvature is the focal 

 distance of the cone of rays emerging from the prism and lens 

 is placed in the focal plane. The rays of different wave-lengths 

 strike it at the same angle and hence are reflected back and 

 recombined in the prismatic. system and brought to a focus at 

 the slit source. Evidently any portion of the spectrum would 

 be recombined here. All screens with such a narrow^ slit 

 were found to diffuse more or less the remaining colours of 

 the spectrum, so that a single narrow mirror was used to 

 reflect a corresponding portion of the spectrum, the remaining 

 portion being allowed to pass on to the darkened walls of the 

 room. After returning through the prismatic system w T here 

 all rays were recombined, a mirror reflected the cone of rays 

 through the polarizing system which come to a focus at the 

 analyser. In this way fields of perfectly uniform tint with 

 vanishing lines for a neutral setting were obtained, and, using 

 the sun's rays, a sensibility for the mean colours greater than 

 that which had previously been possible with the Lippich 

 form using white light. The accompanying table (p. 169) gives 

 a series of successive settings taken at random by Mr. Bates, 

 Fellow 7 in Physics, to whom is due much credit for the 

 perfection of this arrangement and the elimination of the 

 spurious effects in anomalous dispersive substances observed 

 by the other experimenters. 



These give the deviation of a single setting from the mean of 

 from one two-hundred-and- fiftieth to approximately one two- 

 hundreth of a degree for these three wave-lengths. 



In natural rotative substances this system needs no special 

 mounting, but in the study of magnetic rotation it must be 

 free from the action of the field. This has been accomplished, 



* Phil. Mag. [5] xlvi. p. 216 (1898). 



