974 
BEV. T. E, EOBINSON ON SPECTEA OF BLECTEIC LIGHT. 
eacli illuminated element of the slit would receive rays from the whole spark, and the 
spectrum would be a band of uniform brightness in a vertical direction, and terminating 
abruptly above and below. In the actual experiment the length of the spark was 
inch, a quantity comparable with /, being a little less. Hence the spectrum would 
consist of a central stripe of uniform brightness, corresponding to the small portion of 
the slit illuminated by the whole of the spark, accompanied above and below by a ragged 
edge in which the illumination fades away, as more and more of the spark is cut off, till 
nothing is left but the rays coming from its very extremity. The distance from the 
central stripe to which the illumination would be perceived to extend would depend on 
the intiinsic brightness of the part of the spectrum regarded ; and, bearing in mind the 
relative brightness of different parts as laid down by Feaunhofee for the solar spectrum, 
we should get for the visible boundary a curve similar to that represented in the figure. 
—March 1863.] 
Eeceived September 1, 1862. 
On comparing these Tables, it is seen that the jplaces of a very large proportion of 
these lines are identical, or at least differ only within the limits of observation, although 
they may be very unlike in brilliancy and magnitude. Such lines must be considered the 
same ; for the essence of a ray is its wave’s length, and the difference of intensity is but 
an accident depending on extrinsic causes. That this comparison may be more easily 
made, I have given in the following Table the mean places of those whose identity is 
probable : except in the spectra of iron, copper, and many of carbonic oxide, such are 
the great majority of what I have observed. For the conspicuous ones, which I have 
denoted by Greek letters, and Avhich are recognized by peculiar characters, as also for 
some others which stand in definite relation to them, the means were taken without 
strict reference to probable errors; for the rest those limits were taken which I have 
already mentioned. It is possible that in several of the latter class one or two of the 
outliers may not belong to them ; but the general agreement of each is so close as to 
preclude doubt. I include in the Table none beyond Feaunhofee’s H, though several 
were observed, and also omit the very faint red lines which were visible in some spectra 
outside the bright part*. It gives for each the mean place, the extreme deviations from 
that mean, the number of C.P. spectra in which it is found, in how many of these it is 
a *, and the same data for the R. spectra. 
* Especially in manganese, A, C.P. This splendid spectrum, which had 74 lines, is not given, as it was 
not taken in E. or other gases. 
