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POPULAR SCIENCE REVIEW. 
spectrum which have been separated by prismatic analysis, we 
produce white Light. But the statement that a disc painted 
in correct proportions of the primary colours will, if rapidly 
rotated, appear white to the eye, is far from correct. If a 
white circle is left in the centre of such a disc, as represented 
in Fig. 2, it will be seen that the combining colours, when in 
motion, produce, at the greatest velocity, a grey. It must be 
borne in mind that this is not the result of any combination of 
the colours, strictly speaking. The intensity of sensation pro- 
duced by the different rays varies greatly ; and when any 
many-coloured object is in rapid motion, those colours only 
which are the most intense, impress themselves on the retina ; 
and from the persistence of the images so produced, the effect 
obtained is due only to some two or three of them. 
The experiment is ever an interesting one ; it may be studied 
with great advantage by means of Gorham’s colour top, 
which philosophical toy becomes exceedingly instructive, when 
properly employed, in illustrating the phenomena due to the 
blending- of either compound or primary colours in sets, and 
the persistence of one colorn- in close comparison with another. 
To return to the spectrum. Beyond the seven bands of 
colour described by Newton, we may see some others which 
were unknown to him. Having, in our dark room, obtained a 
brilliant spectrum, if it is looked at through a piece of cobalt 
blue glass, a beautiful crimson ray is apparent below the red. 
If the spectrum is thrown on a piece of paper stained yellow by 
turmeric, a lavender grey ray becomes visible beyond the violet. 
If the spectrum is received upon a plate of glass stained of a 
canary yellow by uranium, a pale-green ray is seen beyond the 
lavender grey; and if we throw the spectral rays upon a 
solution of sulphate of quinine, we see celestial blue rays near 
the same spot. The rays last mentioned, have been called by 
Professor Stokes, who has fully investigated their phenomena, 
the fluorescent rays, from the circumstance of their being seen 
in great beauty in many specimens of fluor spar. 
From this statement it would rather appear that the coloured 
rays are more, instead of less, than seven. 
Sir David Brewster has been led, from his extensive in- 
vestigations, to infer that the spectrum which -we see is really 
three spectra overlapping each other ; that the pure colours are 
seen at certain points of maximum effect, as red, yellow, and 
blue. Where the red and yellow blend, orange-coloured light 
is seen ; and where the blue and yellow mix, green is visible. 
It happens, however, unfortunately that we cannot produce green 
by mixing a pure ray of blue Light and a pure ray of yellow Light. 
Under this difficulty many of the continental philosophers con- 
