THE RETURN OF HALLEYS COMET IN 1910. 29 
distance a lump of matter five miles in diameter would appear 
only one-fifteenth of a second across; this would be the very 
tiniest particle that would be separately visible ; smaller particles 
might however be seen as a dusky patch, but only if they are 
closely congregated. There is no chance of seeing any of the 
gaseous envelopes of the comet against the brightness of the 
solar background. Even failure to see anything of the transit 
will teach us something, since we shall be able to fix superior 
limits to the size and density of the particles forming the nucleus. 
Since the tail of a comet points almost exactly away from the 
sun, it was at once seen that there was a possibility of our going 
through the tail at the time of the transit. The only element 
of doubt is whether the length of the tail will be sufficient to 
reach us; it will need to be 15,000,000 miles long, and Dr. 
Holetschek’s researches show that it has only Just attained this 
length at the more recent returns. Even if the tail does reach 
us, 1t is of such ethereal tenuity that it is quite doubtful whether 
we should be able to detect its presence when in the midst of 
it ; there would be no contrast in this case, as when we see it 
from without on the black background of the sky, it would fill 
the whole heavens with a sort of diffused glare; something of 
the kind was recorded when we went through the tail of a comet 
in 1861 (it is instructive to see the apparent form of that 
comet when it was very near the earth; owing to perspective it 
appeared like a widely opened fan; we may look for a similar 
appearance if the tail of Halley’s comet reaches us). Dr. 
Birkeland makes the suggestion that if we pass through the tail 
there may be a striking auroral display; this does not seem im- 
possible, since the aurora is now thought to be due to the excite- 
ment of certain gases in our upper air by electrons emitted by 
the sun, of very similar nature to those supposed to form 
comets’ tails. It is hardly likely that the presence of the tail 
would be sensible in any other way; arrangements have how- 
ever been made by which any abnormal manifestation would be 
fairly sure to be detected. 
So much has iately been written about the physics of comets’ 
tails that it is almost necessary to include some discussion of it. 
There is no question that there is some agency driving the tail- 
particles outwards from the sun much more potently than 
gravitation can pull them in; but as regards the nature of this 
action it is difficult to decide between three contending hypo- 
theses. (i) That it is the pressure of light acting on the very tiny 
particles emitted by the head; this action is quite insensible 
compared with gravity in the case of large bodies, but when the 
