416 ON THE ABSORPTION OF LIGHT BY COLOURED /vIEDIA, 



different homogeneous colours, I naturally had recourse to the 

 prismatic spectrum ; but soon finding the extreme difficulty of 

 procuring a tolerably homogeneous ray by this means, owing 

 to the sun's diameter, the irradiation of the sky, and the im- 

 perfections of prisms, not to speak of the mobility of the spec- 

 trum, being then unprovided with a heliostat, I began to study 

 the resources afforded by transparent coloured media, in hopes 

 that I might discover some, whose limits of transmission, either 

 singly or combined, might be such as to allow the passage of 

 rays only within very narrow limits of refrangibility. In this 

 examination I immediately encountered the singular pheno- 

 mena, noticed, 1 believe, first by Dr Young, of an almost to- 

 tal obliteration of some of the colours by certain glasses, while 

 others intermediate between, or sharply bordering on those 

 obliterated, appeared to be transmitted in all their brilliancy, 

 thus producing an image (when a narrow luminous line is 

 examined with a prism, and such a coloured glass), not con- 

 sisting of a broad band of gradually varying colour, but an as- 

 semblage of more or less sharply defined coloured streaks of 

 different breadths and colours, separated by intervals, in some 

 cases absolutely black, — in others only feebly illuminated. 

 This singularity gave me strong hopes of success in the object 

 of my inquiries, and I was not wholly disappointed. 



3. Among the first specimens of glass which occurred to me, 

 were a disc of bright ruby-red glass, from an ancient window, 

 and a quantity of deep-blue glass, with a sensible tinge of 

 purple, which is manufactured in large quantities for sugar- 

 basins, finger-glasses, &c., and is one of the most common 

 of the coloured glasses. The former permitted to pass almost 

 the whole red, and a considerable portion of the orange ; while, 



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