476 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
cordance with Fraunhofer’s admirable map. These lines are now too well known 
to need description. 
The only map of the fixed lines of the chemical spectrum which I had for a good 
Mdiile 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 Fraunhofer’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 diflScult to 
identify those which are laid down, at le^st 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. 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 in front of the prisms. The breadth of the slit was not 
always quite the same : it may be estimated at about the ^th of an inch. The map 
contains 32 fixed lines or bands more refrangible than H, 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 have 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, O, P have already been appro- 
priated to designate certain fixed lines, I have made use of the small letters I, m, n, 
0 , 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 measurement, except here and there, and they are not, I fear, quite so 
accurate as might be desired ; still, I feel assured that no one viewing the actual 
object would feel any difficulty in identifying the lines with those in my map, pro- 
vided the circumstances under which his spectrum was formed at all approached to 
those under which mine was seen when the arrangement as to focal length of the 
lens, &c. was that most convenient for general purposes. 
The more conspicuous lines in the part of the spectrum represented in the map 
may conveniently be arranged in five groups, which I will call the groups H, /, m,n,p. 
The group H consists chiefly of the well known pair of bands of which the first 
contains Fraunhofer’s line H ; the second band I have marked Tt, in accordance with 
