Dr. Robinson on the Spectra of Electric Light, 487 



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

 difference of colour of discbarges 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 o, 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 

 great difficulty. They vary also in width, from a mere hair's breadth 

 to six or seven times the apparent width of the slit. 



On exhausting the tube in which the discharge is made, at first 

 the only change is that the brilliant lines lose a little of their lustre, 

 till at pressures varying from 3° to o- 5 the spectrum rather suddenly 

 fades away, sometimes leaving only a suspicion of one or two lines ; 

 with others the least-refrangible rays vanish, while the violet remain, 

 though very faint, especially with aluminium. In hydrogen spectra 

 the three bright bands of this gas vanish at unequal densities ; and 

 it is remarkable that this occurs when the gas is diluted to the same 

 proportions by mixing air with it. 



Exhausting yet further, this transition spectrum becomes again 

 bright ; fresh lines appear, and it is changed into a new one, which, 

 however, is never as splendid as that at common pressure, especially 

 at the red end, and in which the very brilliant lines are less frequent. 

 This want makes the difference between the two kinds of spectra 

 seem greater than it really is. Fewer lines are visible in the rarefied 

 media, and of these about four-tenths are not found in the spectra of 

 common pressure. 



If the tables in which the measures are given be examined in re- 

 ference to the points alluded to as doubtful, it will be obvious, 



1 . That many lines are found in all the gases, and in many, perhaps 

 all the metals : the existence of such lines must be independent of 



