114 P, C. on the Coloiirs that enter into the [Aug 



the whole of the green light is excluded, and both the trans- 

 mitted and reflected colour approach very nearly to violet. 



Now, if we include in the combination a single piece 

 of the glass, known as violet glass, but which, in fact, 

 transmits red and violet light in almost equal proportions, 

 the very same effect will be produced by a much smaller 

 number of the blue glasses.'^ The object being to exclude 

 the green light, is accomplished by the interposition of the 

 violet glass, which admits the violet and red in larger 

 proportions than the green : and as the blue glass absorbs 

 the red more readily than it does the green rays, the 

 absence of the latter colour enables it to produce an equal 

 effect with a much less depth of the medium. Hence, the 

 indigo, which represents the blue glass, is economized by 

 interposing the cudbear, which represents the violet, or 

 more properly the crimson glass. 



A mixture of yellow and crimson, upon the same principle, 

 produces red. 



We may illustrate this by another process in dyeing. The 

 natural colour of cochineal is a red inclining to crimson ; in 

 order then to produce scarlet with this dye, a yellow 

 ground is given, either by the mordant, nitro-muriate of 

 tin,t alone, or with the addition of weld, or some other 

 ingredient, depending on the shade required ; the brightest 

 colour, or that which is farthest removed from the natural 

 colour of the dye, being that which requires the greatest 

 depth of yellow. 



This process may be imitated by the combination of a 

 deep crimson (violet) with a yellow glass, such as we 

 have before described : the former of these glasses excludes 

 the green, and the latter the violet light ; consequently the 

 red only is freely admitted : if the green is not intercepted 

 in an equal degree with the violet, and it requires- a con- 



* This may be confirmed by admitting light through an aperture in a card, 

 with the different glasses placed before it in the manner described in another part 

 of this paper. With one blue glass the accidental colour is red ; with several of 

 these glasses, yellow ; with one crimson glass, the accidental colour is green ; 

 but with the addition oio, single blue glass, it is yellow, quite as free from any red 

 tint as when three or four of the blue glasses are applied without tlie crimson (or 

 violet glass.) 



t Nitric acid, diluted with water, gives a beautiful yellow to wool, without 

 any other ingredient. 



