BY THEIR OPTICAL PROPERTIES. 
483 
absorption in blood would negative the supposition 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 
which we are acquainted seems to show that the colouring-matter is some complex com¬ 
pound of the five elements, oxygen, hydrogen, carbon, nitrogen, and iron, which, under 
the action of acids and otherwise, splits into haematin and globulin. 
This example was dwelt on, not for its own sake, but because general methods are 
most readily apprehended in their application to particular examples. To show one 
example of the discrimination which may be effected by the prism, the spectra were ex¬ 
hibited 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 r or green ; but the band 
of the gold glass is situated very sensibly nearer to the blue end of the spectrum than 
that of the copper glass. 
In the experiments actually shown, a battery of fifty cells and complex apparatus 
were employed, involving much trouble and expense. 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, sup¬ 
pose) 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 any one 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, if is not essential that the substance should be in solu¬ 
tion, 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 blood, and the spectrum 
thrown on a w T hite screen. This indicates that the colour of such a paper is in fact due 
to absorption, although the paper is viewed by reflected light. Indeed, by far the greater 
number of coloured objects which are presented to us, such as green leaves, flowers, dyed 
cloths, though ordinarily seen by reflection, owe their colour to absorption. The light by 
which they are seen is, it is true, reflected, but it is not in reflection that the preferential 
selection of certain kinds of rays is made w r hich 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 colourless. The greater part of the light penetrates into the fibres, when it immedi¬ 
ately 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 from, or absorbed by, fibres lying more deeply. At each reflection the various 
kinds of light are reflected in as nearly as possible the same proportion ; but in passing 
across the fibres, in going and returning, they suffer very unequal absorption on the part 
of the colouring-matter, so that in the aggregate of the light perceived the different com¬ 
ponents of white light are present in proportions widely different from those they bear 
to each other in white light itself, and the result is a vivid colouring. 
There are, however, cases in which the different components of white light are reflected 
with different degrees of intensity, and the light becomes coloured by regular reflection. 
Gold and copper may be referred to as examples. In ordinary language we speak of-a 
soldier’s coat as red, and gold as yellow. But these colours belong to the substances in 
two totally different senses. In the former case the colour is due to absorption, in the 
latter case to reflection. In the same sense, physically speaking, in which a soldier’s coat 
is red, gold is not yellow but blue or green. Such is, in faet, the colour of gold by trans¬ 
mission, and therefore as the result of absorption, as is seen in the case of gold leaf, which 
transmits a bluish-green light, or of a weak solution of chloride of gold after the addition 
of protosulphate of iron, when the precipitated metallic gold remains in suspension in a 
finely-divided state, and causes the mixture to have a blue appearance when seen by 
transmitted light. In this case we see that while the substanee copiously reflects and 
intensely absorbs rays of all kinds, it more copiously reflects the less refrangible rays, 
with respect to which it is more intensely opaque. 
All metals are, however, highly opaque with regard to rays of all colours. But cer¬ 
tain non-metallic substances present themselves which are at the same time intensely 
opaque with regard to one part of the spectrum, and only moderately opaque or even 
