328 ON THE CHANGE OF REFEANGIBIL1TY OF LIGHT.
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 fSji< render the vertical sides the longer, but when the whole arrange-
ment of the apparatus is such as to be convenient for observation, the horizontal sides are much longer than the others. In this second spectrum the colours run horizontally, that is to say, the lines of equal colour are horizontal. The interruptions of the primary spectrum corresponding to fixed lines, almost reduced to points, are now elongated, so that in this strangely formed spectrum 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 decomposed 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 horizontally, 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