356 Mr. W. Spottiswoode on [May 21, 



In like manner we may form the Table 



OO+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 illumi- 

 nation, i. e. grey. This effect, however, is partially masked by the fact that 

 the dark bands are not sharply denned like the Fraunhofer lines, but have 

 a core of minimum or zero illumination, 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 x Pj. This may be verified by either using two similar quartz plates 

 Q x Q 1? or by so turning the prism P t that the combination 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 formulas the following : 



00 OEO EO EE 



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



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

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



Again, even when neither of the foregoing conditions are fulfilled, we 

 may still, owing to the breadth of the interference-bands, have such an 

 effect produced that sensibly to the eye 



B+A=B'+A'; 

 and in that case 



B'+A=B+A-A'+A 



=B+A' + 2A-2A>, 



which imply that the images and E may have the same tint, but 

 that E and E E need not on that account be complementary. They 

 will differ in tint in this, that E E, having lost the same tints as E O, 

 will have lost also the tint A, and will have received besides the addition 

 of two measures of the tint A''. 





