1862.] 203 



in its full extent without much more decisive proof than has yet been 

 afforded. It assumes, 



1 . That each substance has a set of lines peculiar to itself. 



2. That those lines are not produced or modified by any molecular 

 agent except heat. 



3. That the spectrum of one substance is in nowise modified by 

 the presence of another ; and in such cases both spectra coexist in- 

 dependency, and are merely superposed. 



4. As may be inferred from 2, that electricity does not make 

 matter luminous directly, but only by heating it ; so that the electric 

 spectrum differs in nothing from that produced by heat of sufficient 

 intensity. 



His attention was directed to this subject several years ago by the 

 difference of colour of discharges in carbonic oxide at common and 

 diminished pressure ; and the results of his experiments appear to 

 show that none of these four points is universally true. 



His apparatus consisted of a powerful induction machine, with 

 which a Leyden jar was connected ; of prisms, first one of 45, after- 

 wards one of 60 (whose deviations were reduced to the scale of the 

 first) ; and of an optical theodolite, in which a collimator with a 

 variable slit gives the beam whose spectrum is observed. He points 

 out an important defect of this arrangement, and discusses the pro- 

 bable liabilities to error proceeding from the graduation reading only 

 to minutes, and from other sources of uncertainty. 



The media of discharge were air, nitrogen, oxygen, hydrogen, and 

 carbonic oxide, to which were added in some instances the vapours 

 of mercury, phosphorus, and bisulphuret of carbon. For electrodes, 

 23 metals and graphite were used 15 with each of the five gases at 

 common pressure and at one of 0< 2, the others only with some of 

 them. In all, 185 different spectra were measured, of which 93 

 were at common pressure. 



At common pressure the spectra show a number of bright lines on 

 a coloured ground, the light of which is in general stronger towards 

 the red than the violet end, and strongest in the green. In some 

 this ground is so bright as to efface all but the most luminous lines : 

 this is especially the case with hydrogen. Of the lines, some are very 

 brilliant ; but they range in light down to the very lowest degree of 

 faintness, such that (at least with the author's apparatus) they can 

 only be seen when the room is entirely dark, and are bisected with 



