Dr. W. M. Watts on the Spectrum of Carbon. 369 



obtained consistently with the smallness of the compass-needles 

 necessary for a thoroughly satisfactory application of the sys- 

 tem of magnetic correctors by which Airy proposed to cause 

 the compass in an iron ship to point correct magnetic courses on 

 all points ? 



L. On the Spectrum of Carbon. By W. Marshall Watts, 

 D.Sc, Physical-Science Master in the Giggleswick School*. 



ALTHOUGH the different comets which have appeared in 

 the northern skies since astronomers have been in pos- 

 session of the spectroscope have been carefully examined by 

 some of the best observers, the exact nature of the comet-spectrum 

 is still a matter of doubt; and the very important question 

 whether comets give the spectrum obtainable from carbon-com- 

 pounds, or only a spectrum of bands of nearly the same refran- 

 gibility, is not decided. 



The explanation of the varying results of different observers is 

 to be found, no doubt, in the extreme faintness of the light 

 emitted by a comet, and the great difficulty of measuring the 

 positions of the lines by any arrangement which requires the 

 bands to be seen together with cross-wires or spider- lines. The 

 best chance of obtaining accurate results is probably to abandon 

 micrometric measurements, and to work by eye-estimations of the 

 distance of the bands from the known bands of some equally 

 faint spectrum, made to occupy the lower portion of the field of 

 view, provided a faint spectrum can be found possessing a suffi- 

 cient number of well-defined bands in the region of the spectrum 

 to be mapped. 



In the case of the comet-spectrum we have just the reference- 

 spectrum required in the well-known spectrum of carbon, which, 

 if it be not identical with the comet-spectrum, has at all events 

 bands of very nearly the same refrangibility, and can easily be 

 obtained of any feeble intensity required. 



It seemed to me therefore of importance to determine the po- 

 sitions of the lines of the carbon-spectrum with as much accu- 

 racy as the spectroscopic means at my disposal allow. 



The spectrum was obtained from the flame of olefiant gas and 

 oxygen, burnt together at the platinum nozzle of an oxyhydrogen 

 blowpipe. 



The spectroscope employed was Browning's automatic spec- 

 troscope of six prisms, with a micrometer eyepiece furnished 

 with two pairs of cross- wires. This eyepiece requires 12*49 

 turns of the micrometer-screw to separate the wires by the in- 



* Communicated by the Author. 

 Phil. Mag. S. 4. Vol. 48. No. 319. Nov. 1874. 2 B 



