568 Mr. Arthur Stanley Eddington [March 26, 



for studying the processes going on in a comet, and the Astronomer 

 Eojal arranged for the taking of long series of photographs at 

 frequent intervals, so as to obtain, as it were, a cinematograph record 

 of the changes taking place throughout the night. The weather was 

 on the whole favourable, and several times a series of eight or nine 

 photographs on a single night was obtained ; in one case the series 

 extended over nearly nine hours, so that there is an opportunity of 

 following step by step the great and striking changes which the 

 comet undergoes in that interval. Altogether the three observers, 

 Mr. Davidson, Mr. Melotte, and Mr. Edney, by dint of constant and 

 continuous watching, obtained 200 j)hotographs ; from these are 

 selected the photographs that are being shown on the screen to- 

 night. 



Besides its position this comet was from another point of view 

 remarkably favourable for our purpose, namely, of studying the 

 motions in the tail. I think that never before have photographs 

 been obtained showing such an amount of intricate detail and such 

 delicate and contorted streamers. This circumstance can best be 

 appreciated by comparing this comet with others previously photo- 

 graphed at Greenwich under precisely similar conditions. In all our 

 previous photographs the streamers are of a straight or gracefully 

 curved character— never contorted, never resembling wavy locks of 

 hair, such as the name " comet " suggests.* But in the present comet 

 the tail is generally composed of fine twisting or curdled streamers ; 

 there are marks which you can seize on, set cross- wires on, and 

 follow through from photograph to photograph. That is just what 

 we require for purposes of investigation. A curious feature is that 

 the head is very faint compared with other comets. It seems clear 

 that this comet had really a very small nucleus, but that for some 

 reason it was extraordinarily explosive, and poured out vast quantities 

 of fine particles to form the abnormally bright tail. The comet was 

 much brighter photographically than visually ; its light was unusually 

 strong at the violet end of the spectrum. 



The part of the comet's tail which is shown on the photographs, 

 taken with the thirty-inch mirror at Greenwich observatory, is about 

 a degree and a half. You can form a fair idea of what that looks 

 like in the sky by remembering that the apparent width of the moon 

 is half a degree ; actually in miles the length of tail shown here 

 would be about four million miles. That sounds a great Ipngth, but 

 on suitable photographs a much greater length of tail could he traced ; 

 we have some photographs taken with an ordinary camera portrait- 

 lens on which a tail 12" long is shown — eight times as much as is 

 shown on the photographs on the screen. As a matter of fact those 

 taken by other observers all over the world are in general on a 

 smaller scale than these Greenwich reflector photographs and show 



♦.. This statement refers to the part of ithe tail close to the head, which is 

 shown in our reflector photographs. 



