394 A Short History of Astronomy [CH. xiil 



issuing from a chimney. This view is confirmed by the 

 fact that the tail is only developed when the comet 

 approaches the sun, a comet when at a great distance from 

 the sun appearing usually as an indistinct patch of nebulous 

 light, with perhaps a brighter spot representing the nucleus. 

 Again, if the tail be formed by an outpouring of matter from 

 the comet, which only takes place when the comet is near 

 the sun, the more often a comet approaches the sun the 

 more must it waste away ; and we find accordingly that the 

 short-period comets, which return to the neighbourhood of 

 the sun at frequent intervals ( 291), are inconspicuous 

 bodies. The same theory is supported by the shape of the 

 tail. In some cases it is straight, but more commonly it is 

 curved to some extent, and the curvature is then always 

 backwards in relation to the comet's motion. Now by 

 ordinary dynamical principles matter shot off from the head 

 of the comet while it is revolving round the sun would 

 tend, as it were, to lag behind more and more the farther 

 it receded from the head, and an apparent backward 

 curvature of the tail less or greater according to the speed 

 with which the particles forming the tail were repelled 

 would be the result. Variations in curvature of the tails 

 of different comets, and the existence of two or more 

 differently curved tails of the same comet, are thus readily 

 explained by supposing them made of different materials, 

 repelled from the comet's head at different speeds. 



The first application of the spectroscope to the study of 

 comets was made in 1864 by Giambattista Donati (1826- 

 1873), best known as the discoverer of the magnificent 

 comet of 1858. A spectrum of three bright bands, wider 

 than the ordinary " lines," was obtained, but they were 

 not then identified. Four years later Sir William Huggins 

 obtained a similar spectrum, and identified it with that 

 of a compound of carbon and hydrogen. Nearly every 

 comet examined since then has shewn in its spectrum 

 bright bands indicating the presence of the same or some 

 other hydrocarbon, but in a few cases other substances 

 have also been detected. A comet is therefore in part 

 at least self-luminous, and some of the light which it sends 

 us is that of a glowing gas. It also shines to a considerable 

 extent by reflected sunlight ; there is nearly always a con- 



