204 ON LIGHT. 



of muriate of chromium, in small thicknesses is green 

 in great ones red ; tincture of violets, and that species of 

 rich blue glass which is coloured with cobalt, in like 

 manner are red when we look through a great thickness, 

 but beautifully blue when thin ; and so in a multitude of 

 other cases. Those who paint in water colours are well 

 aware of what importance it is to effect the tint they aim 

 at by a single wash of their colour. A second applica- 

 tion of the very same liquid, after allowing the first to 

 dry, does not simply heighten the colour, but changes the 

 tint, a circumstance which those who practise that fas- 

 cinating art will do well to bear in mind. 



(48.) When white light is transmitted successively 

 through two or more coloured media whose scales of 

 absorption differ materially, the residual beam, or that 

 which struggles through after passing their successive 

 ordeal, will consist of those rays only whose transmis- 

 sion is favoured by all the media. Hence it will follow, 

 first, that the final tint, or that of the beam ultimately 

 emergent, will most probably be very different not only 

 from that exhibited by either of them separately, but 

 from that which might be expected to arise from a union 

 or blending of their tints, and which won hi arise were we 

 to unite together distinct luminous beams having those 

 tints ; and, secondly, that all such successive transmis- 

 sions tend to produce sombre tints, and ultimately 

 complete blackness ; inasmuch as each successive trans- 

 mission destroys (or absorbs) a greater or less proportion 

 of the total illuminating power of the original beam. 

 Thus when colour is produced on white paper by the 



