ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 285
The only map of the fixed lines of the chemical spectrum which I had for a good while after these researches were commenced is Professor Draper's, which will be found in the twenty-second volume of the Philosophical Magazine (1843). Of course this map cannot be compared for accuracy of detail with Fraun-hofer's map of the visible spectrum, nor does it profess to give more than some of the most conspicuous lines selected from among a great multitude. The suppression of so many lines, without any representation by shading of their general effect, renders it difficult to identify those which are laid down, at least if I may judge from my own observations; besides, Professor Draper's spectrum was so much purer than the one with which I found it most convenient to work, that the two are not comparable with each other.
22. I have made a sketch of the fixed lines from H to the end, which accompanies this paper (see Plate). The fixed lines of the visible spectrum are so well known that I thought it unnecessary to begin before H. A solution of sulphate of quinine is a very good medium for showing the lines, but a yellow glass, which will be mentioned presently, is quite as good, or rather better. The map represents the spectrum as seen with the lens of 12 inches focal length iu front of the prisms. The breadth of the slit was not always quite the same: it may be estimated at about the -c^-th of an inch. The map contains 32 fixed lines or bands more refrangible than //, which is the utmost that I have been able on different occasions to see with this lens, though with a lens of longer focus and a narrower slit the number of fixed lines which might be counted was, as might be expected, a good deal larger. As I have not yet identified these lines, except in certain cases, with those which had previously been represented by means of photographic impressions, I havo thought it advisable not to attempt an identification, but to attach letters to the more conspicuous lines in my map without reference to former maps. As the capitals L, M, N, 0, P have already been appropriated to designate certain fixed lines, I have made use of the small letters I, m, n, o, p, to prevent confusion.
In drawing the map, I have endeavoured to preserve the character of the lines with respect to blackness or faintness, sharpness or diffuseness. The distances were not laid down by measure-
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