452 ON THE ABSORPTION OF LIGHT BY COLOURED MEDIA, 



liering fringe of green. It is quite sufficient for experiments of 

 ordinary delicacy. 



9, It is needless to detail the numerous experiments I have 

 made with glasses of different shades ; but it may not be amiss 

 to mention in a general way, the effect of prismatic analysis 

 on the tints of most usual occurrence in coloured media. Red, 

 scarlet, orange, and brown coloured media, with all the shades 

 of yellow, such as glasses of the above-mentioned hues. Port 

 wine, infusion of saffron, vegetable blues turned red by acids, 

 brandy, India soy, permuriate of iron, muriate of gold, &c. &c. 

 act by extinguishing the violet end of the spectrum with pecu- 

 liar energy, and their type has the general character of Fig. 1. 

 In consequence, they all become red by increase of thickness, 

 and some of them (as Port wine of a good colour), may be 

 used to insulate the extreme red in a high degree of purity 

 and abundance. 



10. Among green media, we may distinguish those in which 

 the type has a maximum ordinate corresponding to some part 

 of the green rays (as in Fig. 6.), and whose hue, in conse- 

 quence, becomes more purely green by an increase of thick- 

 ness. Of this kind are most green glasses, green solutions of 

 copper, nickel, &c. These absorb both the extremities of the 

 spectrum ; the red, with greater energy, if the tint verges to 

 blue, but the violet if to a yellow. Besides these, however, 

 are to be remarked media in which the type has two maxima^ 

 one corresponding to the red, and one to the green rays, as in 

 Fi". 7. In most of these the green maximum is less than the 

 red, and in consequence the tint, on an increase of thickness, 

 passes from green, through a dirty neutral brown, to red. 

 Among these spurious greens, the aqueous solution of sap- 

 green 



