146 Royal Society : — Mr. W. Spottiswoode on 



when viewed in reference to the increase of density due to refri- 

 geration of the material of the shell, to be incapable of producing, 

 during the last 2000 years, any sensible effect upon the length 

 of the day. The author draws various other conclusions, showing 

 the support given by the principal results of this entirely inde- 

 pendent investigation to the verisimilitude of the views contained 

 in his previous memoir. 



May 21.— William Spottiswoode, M.A., Treasurer and Vice-Presi- 

 dent, followed by Dr. Sharpey, Vice-President, in the Chair. 



The following communications were read : — 



" On Combination of Colour by means of Polarized Light." By 

 "W. Spottiswoode, M.A., Treas. and V.P.E.S. 



The results of combining two or more colours of the spectrum 

 have been studied by Helmholtz, Clerk Maxwell, Lord Eayleigh, 

 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, ap- 

 proximating more or less to these, may be produced by chromatic 

 polarization. 



Complementary Colours. — Pirst, as regards complementary colours. 

 If we use a Nicol's prism, N, as polarizer, a plate of quartz, Q, 

 cut perpendicularly to the axis, and a double-image prism, P, as 

 analyzer, we shall, as is well known, obtain two images whose colours 

 are complementary. If we analyze these images with a prism, we 

 shall find, when the quartz is of suitable thickness, that each spec- 

 trum 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 spec- 

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

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

 in one spectrum occupies the position in question, and then to ob- 

 serve the position of the band in the other spectrum. The combina- 

 tions considered in former experiments are those of simple colours ; 

 the present combinations are those of mixed tints, viz. of the parts 

 of the spectrum suppressed in the bands. But the mixture con- 

 sists 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 :— 



