148 Royal Society : — Mr. W. Spottiswoode on 



the Nicol N be turned round, these two overlaps will behave in 

 respect of colour exactly as did the images O and E when Q P 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 for- 

 mulae, it follows that the second analyzer P may be turned round 

 in any direction without altering the tints of the overlaps in question. 

 In like manner we may form the Table 



O O+EE B + A +B +A'=B +A+A'=B 



OE+EO B' + A' + B'+A==B' + A + A'=B'. 



Hence if the Nicol N be turned round, these overlaps will retain 

 their tints ; while if the analyzer P x be turned, their tints will vary, 

 although always remaining complementary to one another. 

 There remains the other pair of overlaps, viz. : — 



OO + OE B+A + B' + A' 



EO+EE B'+A + B+A'. 



Each of these is deprived of the pair of complementaries A, A', 

 B, B'; and therefore each, as it would seem, ought to appear white 

 of low illumination, i.e. grey. This effect, however, is partially 

 masked by the fact that the dark bands are not sharply defined like 

 the Eraunhofer lines, but have a core of minimum or zero illumina- 

 tion, and are shaded off gradually on either side until at a short 

 distance from the core the colours appear in their full intensity. 

 Suppose, for instance, that B' and A' were bright tints, the tint 

 resulting from their suppression would be bright ; on the other 

 hand, the complementary tints A and B would be generally dim, 

 and the image B+A bright, and the overlap B + A+B' + A' would 

 have as its predominating tint that of B + A ; and similarly in other 

 cases. 



There are two cases worth remarking in detail, viz., first, that in 

 which 



B=A', B'=A, 



i. e. when the same tints are extinguished by the combination Q 

 P and by Q l P r This may be verified by either using two similar 

 quartz plates Q x Q x , or by so turning the prism P x that the combi- 

 nation Q x ~P X used alone shall give the same complementary tints as 

 Q P when used alone. In this case the images have for their for- 

 mulae the following : — 



00 OEO EO EE 



A + A' A+A' 2A 2A'; 



in other words, O and E will show similar tints, and E O, E E 

 complementary. A similar result will ensue if B=A, B' = A'. 



