254 TRANSACTIONS OF THE [mAY 17, 



ordiiKiry number of comets, and while ])erliaps no single great 

 step has been made, yet it is certain, I think, that our knowledge 

 of these mysterious objects has gained a real and considerable 

 advance. 



In 1876. curiouslv enough, not a single comet appeared ; but 

 in 1877 there were G; in 1878, 3; in 1879, 5; in 1880, 5; in 1881, 

 8; in 1882, 3; in 1883, 2; in 1884, 3; and in 1885, G; and so far 

 this year, 3. Forty-four comets in all have been observed during 

 the ten years, six of which were conspicuous objects to the naked 

 eye, and two of them, the great cornet of 1881, and the still 

 greater one of 1882, were very remarkable ones. 



The first of tiiese will always be memorable as the first comet 

 ever photographed. Dr. Henry Draper photographed both tlie 

 comet itself and its spectrum; Janssen obtained a ])icture of the 

 comet, and Hugginsof its spectrum. 



A number of excellent iihotograj)hs were obtained of the great 

 comet of 1882, especially by Gill, at the Cape. And it is worth 

 mentioning that in j\Iay, 1882, a little comet (not included in 

 the preceding list, because no observations were obtained of it) 

 was caught upon the photographs of the Egyptian eclipse. 



Two of the bright comets, Wells' comet of 1881 and the great 

 comet of 1882, api)roached very close to the sun, and their 

 spectra, as a consequence, became very complex and interesting. 

 A gretit number of bright lines made their appearance. Sodium 

 was readily and certainly recognized; iron and calcium probably, 

 but not so surely. The evidence as to the nature of the sun's 

 corona, derived from the swift passage of tlie 1881 comet through 

 the coronal regions, has already been alluded to. 



The Pons-Brooks comet of 1883-4 is extremely interesting as 

 presenting the first instance (excepting Halley's comet, of 

 course) of one of the Neptunian family of comets returning to 

 perihelion. There are six of these bodies with periods ranging 

 from sixty-eight to seventy years. Halley's comet, the only 

 large one of the group, has made many returns, and is due in 

 1910. Pons' comet, first observed in 1812, has now returned; 

 Olbers' comet of 1815 is due in 1889, and the three others, all of 

 them small, in 1919-^20 and '22. 



I have spoken of them as Neptunian comets, i. c, their pres- 

 ence in our system is known to be due in some way to this planet. 

 The now generally received theory is that they have had their 

 orbits changed from parabolas into their i)resent state by the 

 disturbing action of Neptune. Mr. Proctor has pointed out 

 certain unquestionable, though, I think, inconclusive, objection^ 

 to this view, and lie proposes, as an alternative, the startling and 

 apparently improbable hypothesis, that they have been ejected 



