460 The Constants of Colour , [October, 
or, if no white light were present, that would also be equally 
apparent. 
Taking all this into consideration, it is evident that when 
a particular colour is presented to us we can affirm that it 
is perfectly pure, viz., entirely free from white light ; or that 
it contains mingled with it a larger or smaller proportion of 
this foreign element. This furnishes us with our first clue 
toward a classification of colours : our pure standard colours 
are to be those found in the spectrum ; the coloured light 
coming from the surfaces of natural objects, or from painted 
surfaces, we must compare with the tints of the spedfrum. 
If this is done, in almost every case the presence of more 
or less white light will be detedled ; in the great majority of 
instances its preponderance over the coloured light will be 
found quite marked. To illustrate by an example : — If 
white paper be painted with vermillion, and compared with 
a solar spedtrum, it will be found that it corresponds in 
general tone with a certain portion of the red space ; but 
the two colours never match perfectly, that from the paper 
always appearing too pale. If, now, white light be added to 
the pure spedtral tint, by refledting a small amount of it 
into the observing-telescope, it will become possible to match 
the two colours, and, if we know what proportion of white 
light has been added, we can afterward say that the light 
refledled from the vermillion consists, for example, of 80 per 
cent of red light from such a region of the spedtrum, plus 
20 per cent of white light. If we set the amount of light 
refledted by white paper as 100, then a surface painted with 
“ emerald-green ” refledts about 8 parts of white light ; arti- 
ficial ultramarine, 2 or 3 parts ; red-lead, 7 or 8, &c. Some 
white light is always present : its general effedt is to soften 
the colour and reduce its adtion on the eye : when the pro- 
portion of white is very large, only a faint reminiscence of 
the original hue remains ; we say the tint is greenish grey, 
bluish grey, or reddish grey. The specific effedfs produced 
by the mixture of white with coloured light will be con- 
sidered hereafter ; it is enough for us at present to 
have obtained an idea of one of the constants of colour, 
viz., its purity. The same word, it may be observed, is 
often used by artists in an entirely different sense : they 
will remark of a painting that it is noticeable for the purity 
of its colour — meaning only that the tints in it have no 
tendency to look dull or dirty, but not at all implying the 
absence of white or grey light. 
Next let 11s suppose that in our study of these matters 
we have presented to us for examination two coloured 
