CHAPTER VII 

 ADDRESSES, REVIEWS, AND CORRESPONDENCE 



IN general Tait did not, like some of his colleagues, begin each session 

 at Edinburgh University with a special introductory lecture upon some 

 chosen subject. Occasionally, however, recent discoveries or new ways of 

 looking at physical problems attracted his attention, and gave an unusual 

 character to the opening lecture. The following two examples will illustrate 

 what is meant. 



In the opening lecture of November 1869 he gave an account of the 

 first great results in solar spectroscopy, and discussed to some extent the 

 nature of nebulae and comets. This was the occasion on which for the first 

 time he gave an explanation of some of the phenomena of comets' tails. 

 Tail's " beautiful sea-bird analogy " as Kelvin called it was also given in 

 a series of interesting articles on Cosmical Astronomy which appeared in 

 Good Words in 1875. The following quotation is taken from a full abstract 

 of the opening lecture just referred to, which was published in Nature, Dec. 16, 

 1869, and was translated in La Revue des Cours Scientifiques, 1870. 



Finally let us consider what we have recently learned about comets bodies 

 which have hitherto puzzled the astronomer quite as much as the nebulae.... There 

 seems to be good grounds for imagining that a comet is a mere shower of stones 

 (meteorites and fragments of iron). This at least is certain that such a shower 

 would behave, in its revolution about the sun, very much as comets are seen to 

 do.... 



Such small comets as have been observed have given continuous spectra from 

 their tails, so far as could be judged with regard to an object so feebly illuminated. 

 This, then, it would appear, is simply reflected solar light. The heads, however, 

 give spectra somewhat resembling those of the nebulae I have just mentioned the 

 spectra of incandescent gases. This is quite consistent with the descriptions given 

 by Hevelius and others of some of the grander comets ; which presented no peculiari- 

 ties of colour in the tail, but where the head was blueish or greenish. Now these 

 appearances are easily reconciled with the shower-of-stones hypothesis. The nucleus, 

 or head, of a comet is that portion of the shower where the stones are most 



