284- Cometary Light. 



both — the existence of elementary or unborrowed light. This 

 indeed was nothing new ; it had been maintained by ^ and 

 Schroter in 1807 and 1811, though subsequently discredited by 

 other authorities. Bat the very curious distinction between the 

 light of the central body and its envelope, though previously 

 indicated by polarization experiments, was reserved to be 

 demonstrated by the spectroscope of Huggins. Schroter, the 

 strenuous assertor of electric light in comets, was so unsuspi- 

 cious of this distinction that some of his chief proofs were drawn 

 from the rapid changes and coruscations of the tail. On the 

 other hand, Arago, the great advocate of reflected light, was 

 somewhat less confident. His examination of the comet of 1819 

 with a doubly refracting prism showed two images of somewhat 

 unequal intensity indicating polarization, and consequently 

 reflected light ; and at the return of Halley's comet in 1835 he 

 obtained more decided evidence by the interchangeably red 

 and green colours of the images in his polariscope, and thus he 

 satisfied himself that the light of the body was not, at least in 

 general, native, though he is very cautious in his conclusion, 

 candidly admitting that the whole light might be partly intrinsic 

 and partly reflected, since bodies becoming incandescent do not 

 thereby lose the power of reflection. From this, however, 'it 

 appears that he had less idea of the really native character of 

 the light of the nucleus than of its becoming enkindled by its 

 approach to the sun. In the same manner Bruhns at Berlin 

 in 1858, and Murmann at the Vienna Observatory in 1862, 

 found that the light of the great comets of those years, taken 

 as a mass, was polarized, and therefore reflected. 



In 1861 Secchi had obtained a better result from the mag- 

 nificent apparition of that summer. He found the polarization 

 of the tail and rays near the nucleus very strong, but no trace 

 of it in the nucleus till July 3 and following days, when it 

 presented decided indications of it, notwithstanding its very 

 minute size (hardly 1", July 7). This he thought a fact of 

 great importance, as showing that the nucleus had possessed 

 native light on the former days, being perhaps rendered 

 incandescent in its perihelion. The following year brought us 

 a less brilliant but curiously developed comet, which has 

 been described at length in one of our earlier volumes, and in 

 this again he could detect no polarization in the nucleus, 

 except a feeble trace on the last day of observation, while the 

 aigrettes, or jets, unpolarized at their origin, became more so 

 as they passed into the strongly polarized condition of the 

 surrounding nebulosity. Non-polarization, however, is no con- 

 clusive evidence of native light, though polarization is a test 

 of reflection ; and hence Secchi supposed that the nucleus and 

 aigrettes, if not incandescent, which he thought a less probable 



