OF SOME OF THE CHEMICAL ELEMENTS. 
143 
The spectrum of comparison was received by reflexion from a prism placed in the 
usual manner over one-half of the slit. As the spectrum of the discharge between 
points of platinum, when these are not too close, is, with the exception of two or 
three easily recognized lines, a pure air-spectrum, this was usually employed as a con- 
venient spectrum of comparison for distinguishing those lines in the compound spectrum 
which were due to the particular metal employed as electrodes. The measures, however, 
of all the lines, including those of the air-spectrum itself, were invariably taken from 
the light received into the instrument directly, and in no case has the position of a line 
been obtained by measures of it taken in the spectrum of the light reflected into the slit 
by the prism. 
The measures of all the lines were taken more than once ; and when any discordance 
was observed between the different sets, the lines were again observed. The spectra of 
most of the metals were re-measured at different times of observing. In the measure- 
ment of the solar lines for their coordination with the standard air-spectrum, the 
observations were repeated on several different occasions during the progress of the 
experiments. The line G of the solar Table is the one so marked by Kirchhoff*. 
When no change in the instrument could be detected, the measures came out very closely 
accordant, for the most part identical. The discordances due to small alterations in the 
instrument itself were never greater than 5 or 6 of the units of measurement in the 
whole arc of 4955 units. As the apparatus remained in one place free from all appa- 
rent derangement, these alterations are probably due to changes of temperature. The 
method employed to eliminate these discordances has been described. 
Throughout the whole of the bright portion of the spectrum the probable error of 
the measures of the narrow and well-defined lines does not, I believe, exceed one unit 
of the scale. 
In the case of lines of sensible breadth and of nebulous bands, the point of intersection 
of the wires of the micrometer was brought as nearly as possible upon the centre of the 
lines. 
7. It is well known that the lines of different metals as a whole, as well as the lines 
of the same metal amongst themselves, differ greatly in their characters. For example, 
the narrow sharply-defined lines of cobalt and iron contrast strikingly with the broader 
and nebulously edged lines of antimony and arsenic. The spectrum of zinc affords a 
good example of the differences in this respect between lines of the same metal. In 
general, it may be that the less volatile a metal is, the narrower and more sharp are 
the lines, though indeed in the case of the metals barium, calcium, and strontium 
many of the lines are of hair-like narrowness and sharply defined. 
In the spectra of many of the metals bands of light also exist, generally rather broad 
and faint, which are not resolvable with my instrument into lines. Many of these have 
the appearance of being true nebulous bands, whilst others under careful scrutiny 
present indications of being probably composed of lines. 
* Untersuchungen ii. d. Sonnenspectrum, 2 Theil, Taf. iii. Berlin, 1863. 
