PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 507 
points. If, now, the whole be viewed through a prism, so as to be refracted in a verti- 
cal plane, the effect is very striking. For facility of explanation suppose the red to 
be to the left, and the rays to be refracted upwards, so that to the observer the image 
is thrown downwards. The original spectrum on the screen is decomposed by the 
prism held to the eye into two spectra, which diverge from each other. The first of 
these runs obliquely downwards from left to right, and contains the natural colours 
of the spectrum from red to violet. It consists of light which has been scattered in 
the ordinary way by the substance on which the primary spectrum is received, and 
the cause of its obliquity is evident. The second spectrum is horizontal, that is to 
say, it approximates to the form of a long rectangle having its longer sides horizontal. 
Of course it would be theoretically possible to render the vertical sides the longer, 
but when the whole arrangement of the apparatus is such as to be convenient for ob- 
servation, the horizontal sides are much longer than the others. In this second spec- 
trum the colours run horizontally^ that is to say, the lines of equal colour are hori- 
zontal. The interruptions of the primary spectrum corresponding to fixed lines, 
almost reduced to points, are now elongated, so that in this strangely formed spec- 
trum the principal fixed lines of the solar spectrum are seen running across the 
colours. 
101. It will be convenient to have a name for the second of the two spectra above 
mentioned. As the term secondary spectrum is already appropriated to something 
altogether different, I shall call it the derived spectrum. The first of the diverging 
spectra may be called the primitive spectrum, while the original spectrum, considered 
as not yet deconriposed by the prism held to the eye, may be called, for distinction, 
as in fact it has been already called, primary. 
102. In accordance with the law enunciated in Art. 80, it is found that the derived 
spectrum appears always on one and the same side of the primitive, being less refracted. 
103. The brilliancy of the derived spectrum, its extent, both vertically and hori- 
zontally, the colours of which it mainly consists, the distribution of its illumination 
in a horizontal direction, all depend upon the nature of the substance upon which 
the primary spectrum is received. As a general rule, it may be stated that it starts 
from the neighbourhood of the brightest part of the primitive spectrum, and extends 
from thence onwards to a good distance beyond the extreme violet ; and that with a 
given substance its colour is pretty uniform, that is, does not much change in passing 
from one vertical section to another. Sometimes the derived spectrum remains very 
bright up to its junction with the primitive, or at least till it gets so near that the 
superior brilliancy of the primitive spectrum prevents all observation on the derived ; 
sometimes it remains dull to a considerable distance from the primitive spectrum, 
and then, opposite a highly refrangible part of the primitive spectrum, a strong illu- 
mination comes on in the derived, lasts for some distance, and afterwards gradually 
dies away. Many of the results mentioned in this paragraph are better observed by a 
somewhat different method, which will shortly be described. 
