LiOfiHT AND COLOUR. 
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the Earth and travel towards the Sun, and be enabled to 
maintain this speed through the entire space between us, we 
should require more than 108 years to complete the journey, 
and yet a ray of Light passes over the same space in seven 
minutes and a half. 
If the reader will place himself in a dark room, of course 
objects are invisible to him; there is a negation of colour, because 
there is an absence of Light. Pierce the shutter with a needle, 
a thread of Light flows into the room, forms a round luminous 
spot of white Light on the floor; from this point radiation is 
established, and the eye begins to distinguish objects and to 
be sensible of colour. The phenomena of vision, and the 
phenomena of colour, are therefore proved to be dependent on 
the presence of the sun-beam. 
If we take a triangular piece of glass (a prism), and place it 
so that the Sun-ray falls upon one of its angles, a new pheno- 
menon results. A ray of Light, in passing from any medium 
into another of a different density, is bent out of its path, or 
undergoes refraction. The degrees of refraction are dependent 
upon the varying thickness of the mass, so that our wedge of glass 
presents a gradually thickening transparent medium to the rays. 
By this the Sun-beam, which enters the prism, of the most trans- 
parent whiteness, or colourless, leaves it a sheaf of rays intense in 
the brilliancy of their varied colours. The spectrum so formed, 
if received upon a screen, will be of a flame-like shape, and, 
commencing at that end where the ray is least bent, we have 
the folloAving order in our chromatic scale : red, orange, yellow, 
green, blue, indigo, and violet. This image (which was repre- 
sented in our last number, in connection with the dark lines by 
which it is crossed) has been called the Newtonian Spectrum, 
for to Newton we owe the first careful investigation of the 
phenomena. That philosopher determined the number of 
primary rays to be, as above enumerated, seven. Sir David 
Brewster, however, reduces the number to three : red, blue, 
and yellow; and from these he argues that all the infinite 
variety and beauty of colour which we see in nature must be 
derived by intercombination. Fig. 1 represents the principle 
of this ; the three primary colours are in the corners of the 
equilateral triangle ; green, orange, and violet, are produced 
by the combinations of the nearest two of these, and the neutral 
tint of the centre is the result of combining the three colours. 
This appears to require that a very prevalent error should be 
noticed. It is often stated that by combining pigments of the 
colours of the spectral rays, white is produced. Nothing can 
be further from the truth, as Hade is the result of the 
experiment. 
It is undoubtedly time that if we recombine the rays of the 
