338 
POPULAR SCIENCK REVIEW. 
all directions from the white-hot charcoal. If the charcoal be 
enclosed in a dark lantern, I can allow a portion only of its 
light to escape into the room, and can direct it at pleasure into 
any part by using a small mirror or flat polished surface. The 
opening by which the light escapes is, in this instance, a narrow 
vertical slit. You will observe the light is of a pure white. 
I propose now to show you another property of light, and to 
prove that white light consists of a mixture of several different 
colours. If the slice of light which issues from the lamp be 
allowed to fall upon a clear plate of glass with flat faces parallel 
to each other, the light will pass through the glass without 
undergoing any apparent change either in its colour or its 
direction ; but if it be allow^ed to fall upon one of the faces of a 
piece of glass cut into the form of a triangular bar or prism, we 
shall have a very different result. The light will be abruptly 
altered in its direction as it passes through the glass ; it will be 
refracted, as it is said : and now the beam of light, instead of 
falling upon the screen as a slice of white light, will be spread 
out into a ribbon of gorgeous tints, the brillianji hues of which 
will graduate insensibly from red into violet. This is repre- 
sented in fig. 2 of the Plate. The red end of the beam of light 
which is least altered from its original direction is said to be 
the least refrangible ; whilst the violet, which has experienced 
the greatest change, is said to possess the greatest amount of 
refrangibility. As this word “ refrangibility ” is one which I 
shall often have to use, it is necessary that you should dis- 
tinctly understand what it means — viz. the degree to which any 
ray is suddenly bent from its original direction by the action of 
the prism. 
Such a coloured image constitutes what Sir Isaac Newton 
called the prismatic spectrum. He varied this experiment in a 
great number of ways, and concluded that white light consists 
of a mixture of various colours, like those of the rainbow. By 
recombining these colours, the original white light is repro- 
duced. This may be done by sending it through a second 
prism placed in tlie opposite direction to the first. The action 
of the prism* which we have just examined is to open out the 
colours of which the white light consists into a fan of coloured 
light; so that, instead of perceiving a single white image of the 
slit, a series of images is obtained of every shade of colour. 
Koc'h image possesses its own special degree of refrangibility, 
and its characteristic tint; whilst each overlaps its neighbour on 
either side, so that the whole forms a continuous and beautiful 
blending of harmonious hues, commencing with red and ending 
in the violet. What the pitch of a note is in sound, such is 
colour in light. The undulations of the ether are longest and 
slowest in the red, and sliortest and most rapid in the violet. 
