354 Mr. W. Spottiswoode on [May 21, 



III. " On Combinations of Colour by means of Polarized Light/' 

 By W. SPOTTISWOODE, M.A., Treas. & V.P.R.S. Received 

 April 8, 1874. 



The results of combining two or more colours of the spectrum have 

 been studied by Helmholtz, Clerk Maxwell, Lord Rayleigh, and others ; 

 and the combinations have been effected sometimes by causing two 

 spectra at right angles to one another to overlap, and sometimes by 

 bringing images of various parts of a spectrum simultaneously upon the 

 retina. Latterly also "W. v. Bezold has successfully applied the method 

 of binocular combination to the same problem (Poggendorff, Jubelband, 

 p. 585). Some effects, approximating more or less to these, may be pro- 

 duced by chromatic polarization. 



Complementary Colours. First as regards complementary colours. If 

 we use a Mcol's prism, N, as polarized, a plate of quartz, Q, cut per- 

 pendicularly to the axis, and a double-image prism, P, as analyzer, we 

 shall, as is well known, obtain two images whose colours are comple- 

 mentary. If we analyze these images with a prism, we shall find, when 

 the quartz is of suitable thickness, that each spectrum contains a dark 

 band, indicating the extinction of a certain narrow portion of its length ; 

 these bands will simultaneously shift their position when the Nicol N is 

 turned round. Now, since the colours remaining in each spectrum are 

 complementary to those in the other, and the portion of the spectrum 

 extinguished in each is complementary to that which remains, it follows 

 that the portion extinguished in one spectrum is complementary to that 

 extinguished in the other ; and in order to determine what portion of 

 the spectrum is complementary, the portion suppressed by a band in any 

 position we please, we have only to turn the Nicol N until the band in 

 one spectrum occupies the position in question, and then to observe the 

 position of the band in the other spectrum. The combinations considered 

 in former experiments are those of simple colours ; the present com- 

 binations are those of mixed tints, viz. of the parts of the spectrum sup- 

 pressed in the bands. But the mixture consists of a prevailing colour, 

 corresponding to the centre of the band, together with a slight admixture 

 of the spectral colours immediately adjacent to it on each side. 



The following results, given by Helmholtz, may be approximately 

 verified : 



Complementary Colours. 



Bed, Green-blue ; 



Orange, Cyanic blue ; 



Yellow, Indigo-blue. 



Yellow-green, Violet. 



When in one spectrum the band enters the green, in the other a band 

 will be seen on the outer margin of the red and a second at the opposite 



