Discrimination of Organic Bodies by their Optical Properties. 391 



£The spectrum of sulphocyanide of peroxide of iron was shown, for 

 . the sake of contrasting with blood. 3 Hence the appearance of such 

 ■ peculiar system of bands of absorption in blood would negative the 

 apposition that its colour is due to a salt of iron as such, even had 

 we no other means of deciding. The assemblage of the facts with 

 'nch we are acquainted seems to show that the colouring matter is 

 ;e complex compound of the five elements, oxygen, hydrogen, 

 ,rbon, nitrogen, and iron, which, under the action of acids and 

 iherwise, splits into hsematine and an albuminous substance. 

 a This example was dwelt on, not for its own sake, but because 

 general methods are most readily apprehended in their application 

 j particular examples. To show one example of the discrimination 

 which may be effected by the prism, the spectra were exhibited of 

 the two kinds of red glass which (not to mention certain inferior 

 kinds) are in common use, and which are coloured, one by gold, and 

 the other by suboxide of copper. Both kinds exhibit a single band 

 of absorption near the yellow or green ; but the band of the gold 

 glass is situated very sensibly nearer to the blue end of the spectrum 

 han that of the copper glass. 

 In the experiments actually shown, a battery of fifty cells and 

 complex apparatus were employed, involving much trouble and ex- 

 pense. But this was only required for projecting the spectra on a 

 screen, so as to be visible to a whole audience. To see them, nothing 

 more is required than to place the fluid to be examined (contained, 

 suppose, in a test-tube) behind a slit, and to view it through a small 

 prism applied to the naked eye, different strengths of solution being 

 tried in succession. In this way the bands may be seen by anyone 

 in far greater perfection than when, for the purpose of a lecture, they 

 are thrown on a screen. 



In order to be able to examine the peculiarities which a substance 

 may possess in the mode in which it absorbs light, it is not essential 

 that the substance should be in solution, and viewed by transmission. 

 Thus, for example, when a pure spectrum is thrown on a sheet of 

 paper painted with blood, the same bands are seen in the yellow and 

 green region as when the light is transmitted through a solution of 

 yood, and the spectrum thrown on a white screen. This indicates 

 hat the colour of such a paper is in fact due to absorption, although 

 ne paper is viewed by reflected light. Indeed, by far the greater 

 Lumber of coloured objects which are presented to us, such as green 

 leaves, flowers, dyed cloths, though ordinarily seen by reflexion, owe 

 their colour to absorption. The light by which they are seen is, it 

 is true, reflected, but it is not in reflexion that the preferential selec- 

 tion of certain kinds of rays is made which causes the objects to 

 appear coloured. Take, for example, red cloth. A small portion of 

 the incident light is reflected at the outer surfaces of the fibres, and 

 this portion, if it could be observed alone, would be found to be co- 

 lourless. The greater part of the light penetrates into the fibres, 

 when it immediately begins to suffer absorption on the part of the 

 colouring matter. On arriving at the second surface of the fibre, a 

 portion is reflected and a portion passes on, to be afterwards reflected 



