An introductory text-book of logic - Mellone, Sydney - Rare Books at ...

OR SCIENTIFIC METHOD. 275
image of the sun (negative instance). But on passing the
beam through a prism, the image becomes nearly five times
as long as it is broad, and is coloured from end to end by a
succession of vivid tints (positive instance). Hence some
is the cause of the colours. Newton now
thing in the glass
proceeded to vary the size of the prism, to vary the quality of
the glass, to pass the beam through different parts of the
same prism, and to try other minor suppositions ; but none
of these changes made any difference in the colours. Hence
he concluded that the prismatic shape of the glass was the
real cause. He eliminated this by placing on the original
prism a second one of exactly the same angle, but inverted,
so that together the two prisms formed a solid with parallel
surfaces. The light, passing through both, came out un-
coloured and gave a perfect undistorted image of the sun.
Hence the prismatic shape of the glass was proved to be the
cause of the colours. Newton now adopted the idea that
white light is really compound, being composed of differentlycoloured
primary rays, each undergoing a different degree of
refraction (change of direction on passing into the glass of
the prism). So he proceeded to test the actual properties
possessed by each ray separately. Through a hole in the
screen, any one ray could be transmitted while the rest were
stopped. The transmitted ray was passed through a second
prism, and was found to undergo only a change of direction.
When this was done to each of the distinct coloured rays, the
latter were found to be refrangible by the second prism in
different degrees the violet most, the red least ; precisely,
that is, in the same order as by the first prism in forming the
elongated spectrum. Thus the composite character of white
light was proved, and the fact that the primary rays compos
ing it have different degrees of refrangibility corresponding
to the differences of colour. 1
The student should notice that in every case the
Method is applied in order to test a suggested cause;
although the suggestion does not always arise from
a deliberate application of the Method of Single
Agreement.
1 Cf. Baden - Powell s History of Natural Philosophy, p. 279.