1909] on Recent Results of Astronomical Research. 569 



the outer parts of the tail ; so far as I know there are no other 

 series of photographs similar to these, which are specially adapted for 

 studying the fine detail near the head of the comet, and what may 

 be called the first eighth of its tail. Probably those which approach 

 to them most nearly are some by Professor Wolf at Heidelberg, which 

 are on half the scale. Yet one other fact I may mention ; owing 

 to the great liglit-gathering power of the reflector fine photographs 

 of the comet could be obtained with quite short exposures ; this is a 

 matter of great consequence. At first we took some plates with 

 exposures of half to three-quarters of an hour as well as shorter 

 ones ; but it was found that exposures of ten to fifteen minutes 

 showed finer detail and were better for our purposes. This was not 

 because the longer ones were over-exposed, but because the changes 

 taking place within the comet are so rapid that in a thirty-minutes 

 exposure all the fine lines and streamers become blurred. 



I hope it will be understood that I mention these features of the 

 Greenwich photographs, not with any idea of claiming that they are 

 superior to or more important than those taken by many observers 

 elsewhere ; to do so would not do justice to the remarkable results 

 obtained by Barnard, Wolf and others ; it is rather to show that 

 these photographs cover new ground, and the evidence they afford 

 and the results they lead to will be somewhat different from those 

 elsewhere. And for the reasons already explained this is the first 

 time it has been found possible to thoroughly examine a comet in 

 this way. As regards the results, we are still in the midst of the 

 work of examining the photographs, a great deal of preliminary 

 computation had to be gone through before a serious start could be 

 made ; the measures of the details require very much comparing and 

 digesting. By the kindness of the Astronomer Royal I am permitted 

 to speak of them so far as we have progressed at present, but all I can 

 say must necessarily be of a very preliminary character. 



Let us first turn attention to the intermittent character of the 

 activity of the comet. The outpouring of matter to form the tail is 

 not at all steady, but periods of great disturbance and quiescence 

 succeed one another at intervals of a few days. The eruption goes 

 through stages which are more or less well defined. [A typical series 

 of slides showing the stages of the eruption was shown.] 



But the great outstanding problem in the study of comets, the 

 one on which more particularly we hope to obtain some light by 

 the taking of this extensive series of photographs, is the study of the 

 motions of the particles of the tail and the forces which cause them. 

 The tail streams out from the comet in the direction nearly directly 

 away from the sun. It is not simply left behind by the comet. It 

 is driven away from the sun, and may be either in front or behind 

 the head in its orbital motion according to circumstances. We are 

 accustomed to regard the sun as the centre of an attractive force by 

 which the planets are kept in their orbits, and comets' heads also 



