1874.] Colour-combinations by Polarized Light. 355 



end of the violet showing that to the green there does not correspond 

 one complementary colour, but a mixture of violet and red, i. e. a reddish 

 purple. 



Combination of two Colours. Next as to the combination of two parts 

 of the spectrum, or of the tints which represent those parts. If, in 

 addition to the apparatus described above, we use a second quartz plate, 

 Q, and a second double-image prism, P t , we shall form four images, say 

 O, O E, E 0, E E ; and if A, A' be the complementary tints ex- 

 tinguished by the first combination QP alone, and B, B' those ex- 

 tinguished by the second Q t P x alone, then it will be found that the fol- 

 lowing pairs of tints are extinguished in the various images : 

 Image. Tints extinguished. 



00 B, A, 



OE B', A', 



E O B', A, 



EE B, A'. 



It is to be noticed that in the image E the combination Q : P : has 

 extinguished the tint B' instead of B, because the vibrations in the 

 image E were perpendicular to those in the image O formed by the com- 

 bination Q P. A similar remark applies to the image E E. 



The total number of tints which can be produced by this double com- 

 bination Q P, Qj Pj is as follows : 



4 single images, 

 6 overlaps of two, 

 4 overlaps of three, 

 1 overlap of four. 



Total.. 15 



Collateral Combinations. The tints extinguished in the overlap 

 OO+EO will be B, A, B', A; but since B and B' are complementary, 

 their suppression will not affect the resulting tint except as to intensity, 

 and the overlap will be effectively deprived of A alone ; in other words, 

 it will be of the same tint as the image O would be if the combination 

 Q, P x were removed. Similarly the overlap OE + EE will be deprived 

 effectually of A' alone ; in other words, it will be of the same tint as E, 

 if Q! Pj were removed. If therefore the Nicol N be turned round, these 

 two overlaps will behave in respect of colour exactly as did the images 

 and E when QP was alone used. We may, in fact, form a Table 

 thus : 



Image. Colours extinguished. 



OO + EO B + A+B'+A=B + B'+A=A 



OE + EE B' + A' + B +A' = B + B' + A'=A'. 



And since the tints B, B' have disappeared from each of these formulae, 

 it follows that the second analyzer P may be turned round in any direc- 

 tion without altering the tints of the overlaps in question. 



