1882.] 



On the Spectrum of Carbon. 



407 



slide was turned round through 180° about the axis of the telescope, so 

 as to bring to the top that part of the sensitive plate which had been 

 before lowest. It was then exposed a second time, and thus two images 

 of the same line were impressed on the plate, which were necessarily 

 at equal distances on either side of the point where the axis of the 

 telescope met the plate. By a subsequent measurement with a 

 micrometer under a microscope of the distance between the two 

 images, and the conversion of this distance into angular measure, a 

 correction was found, which was added to, or subtracted from, the 

 reading of the circle to get the exact deviation of the ray producing 

 the line under observation. Another photograph of the same line was 

 next taken, in the same way as before, except that the telescope was 

 placed at the corresponding angle on the other side of the collimator. 

 From the two angles thus found, the wave-length of the line was 

 calculated. The process was repeated three or four times for each 

 line, and the mean wave-length thus found for carbon lines were 

 2296-5,- 2478-3, 2509'0, 2511-9, 2836-3, and 2837'2. The numbers 

 deduced from the different photographs of the same line differed from 

 one another in the last figure only, so that we are justified in assuming 

 the first four figures to be accurate in each case. The wave-lengths 

 of the remaining lines were obtained by interpolation from measures 

 of photographs taken with a train of two calcite prisms of 30° each, 

 and one of 60°, on which the iron as well as the carbon lines were 

 shown. The wave-lengths of the iron lines used in the interpolations 

 were deduced from photographs taken with the grating in the same 

 way as that above described for the carbon lines. The wave-lengths 

 thus found for the remaining carbon lines are given in the table 

 below. 



In taking the photographs of the spark, the induction coil was 

 sometimes worked by a De Meritens magneto-electric machine, and in 

 that case the stream of sparks was not only extremely brilliant, but 

 produced a deafening roar. Notwithstanding this character of the 

 spark, the photographs, when the spark was taken in air, between 

 poles of purified graphite, showed, besides the carbon lines above 

 described, the set of six cyanogen flntings in the blue very distinctly, 

 and those between K and L, and those near N, strongly developed. 

 On the other hand, when the spark was taken in carbonic acid gas, 

 these flutings almost entirely disappeared, and would no doubt ha ve 

 •disappeared entirely, if the last traces of air had been removed from 

 the apparatus. 



