76 
POPULAR SCIENCE REVIEW. 
scale of light and shade than the painter in oils or water-colours. In speaking, 
next of colour-production by dispersion, the use of the prism is explained, 
and the infinite number of gradations into which white light can thus be 
broken up ; the positions of the coloured spaces, and the proportion occupied 
by each, is estimated according to original observations, — blue and blue-violet 
representing 311 ; violet, 194 ; red, 149 ; green, and yellow, and yellowish- 
green, 104 ; green and blue-green, 103 ; and all the others far smaller areas, 
raising the total to 1000 parts. The irrationality of the prismatic spectrum 
is contrasted with the normal length as obtained by means of a diffraction- 
grating, red here standing for 330 parts, violet-blue for 117, orange-red for 
104, blue for 74, and the rest for the same complement; yellow here occu- 
pying the centre of the spectrum, the red and orange being considerably 
increased to the detriment of the blue and violet. It seems that the eye is 
far more sensitive to changes of wave-length in the middle regions than at 
either extremity. This is considered to negative theories of colour founded on 
supposed analogies with music. 
The Constants of Colour are next adverted to, and the mode of mixing 
white light with the colours of the spectrum described. Most coloured sur- 
faces are shown to be more or less adulterated with white light, and in 
many instances the latter even preponderates. The first Constant is therefore 
'purity, or absence of white. The second is a photometric problem, namely, 
relative brightness, or luminosity. Here an ingenious system of rotating 
discs is brought into play, in which mixtures of colour in the form of sectors 
are estimated against proportional combinations of simple black and white. 
Black can hardly be looked on as an absolute negation, since it reflects from 
two to six per cent. Allowing for this, if white paper be represented by a 
luminosity of 100, chrome yellow has one of 803, emerald green of 48’6, 
cobalt blue of 35 ‘4, vermilion of 25*7, and ultramarine of 7*6. The hue of 
colour is the third and last Constant ; this depending on wave-length and 
position in the normal spectrum. The production of Colour by Interference 
and Polarization occupies the fourth chapter, the latter never being seen 
out of the laboratory, while the former furnishes the brilliant plumage of the 
peacock, the humming-bird, and the iridescent armour of beetles. The colours 
of opalescent media are next studied, as seen in milky solutions, in smoke, and, 
on the grandest scale, in the sky. 
Fluorescence and Phosphorescence are next discussed, and the more 
essentially artistic topic of Colour by Absorption is reached. To this process 
the colours of ordinary objects are almost entirely due. This is studied by 
the spectroscope in tinted glass, and diagrams are given, showing the very 
complex nature of the waves transmitted by red, orange, green, and blue 
specimens. A hand-spectroscope suffices to demonstrate the same law in 
blue paper, which, besides white light, giving a continuous spectrum, reflects 
red, yellow, blue, and violet. The white reflected does not, in a strong 
colour, exceed that just named as emanating even from black. In velvet, an 
attempt is made to suppress all surface light, and to display only that which 
has penetrated deeply and thus become highly coloured. The green of vege- 
tation offers a peculiar case. Extreme red is present, with orange, yellow, 
and yellowish-green, running into a full green. A mixture of red and green 
is, later on, shown to furnish yellow light. 
