POLARISED LIGHT EFFECTS BY DIFFRACTIOX. 



409 



obtain an admixture of two or more pure spectral colours in the 

 natural propoo'tions present in white light. 



If we have a screen with two adjustable slots, we can at will 

 cause any part of the spectrum of one to overlap the spectrum of 

 the other, and by having the slots A and B arranged the one a 

 little above the other (Fig. 2) the top or bottom of the field of view 

 shows the two colours separately, which in the centi-al part of the 

 field form the admixture. 



Of course there is nothing novel in mixing spectral colours ; the 

 only point about this way of doing it is its great simplicity, and 

 the fact that, since both spectra are formed by the same grating 

 and with the same source of light, provided the two slots are of 



Fig.2. 



equal width, the amount of the colours mixed will be in the exact 

 natural proportion present in the light which is being used. 



A screen having two slots, which I will place under my micro- 

 scope later in the evening, shows two separate spectra, nearly, but 

 not quite, overlapping in such a way that their outer violet edges 

 show on both sides. Almost the whole of. the overlapping part 

 shows as a canary yellow. 



I have ventured to bring these experiments before you, in the 

 belief that in their present form they ai-e new ; if not, it will be 

 interesting if Mr. Nelson or some other gentleman will kindly 

 inform us on the subject. 



Since the above was written I have found that the arrange- 

 ment of slots extending one above the other so as to show at a 

 glance an admixture of colours and the separate spectral colours 



