1909] on Recent Results of Astronomical Research. 573 



streaming away from the sun to the effect of hght-pressure. When 

 radiation of any kind, sunUght or the heat from a fire falls on a 

 surface, it exerts a pressure on that surface tending to drive it back. 

 The light from the lantern which was falling on the screen just now 

 presses against the screen, though the whole pressure is exceedingly 

 minute ; the whole pressure of sunlight falling on the earth is some- 

 thing like 150,000 tons weight, a force which is insufficient to make 

 the earth budge so much as one hair's breadth from its path. But 

 on the small particles of a comet's tail its effect may be of importance, 

 because although the force of pressure decreases with the size of the 

 particle, it does not do so so rapidly as the volume or weight, so that 

 the effect on the motion is up to a certain point greater the smaller 

 the particle. In the case of a particle 2-0^0 o i^i- i^ diameter, the 

 hght-pressure would just about balance gravitation ; such a body 

 would be neither attracted to nor repelled from the sun. For one 

 whose diameter is jo oVo i^-' ^^^ repulsion of light-pressure would be 

 twenty times gravitation. You will remember that Bredichin found 

 three classes of tails, of which the one most powerfully repelled 

 the repulsion was 18 times gravitation. We have only to suppose 

 that the particles are of this order of magnitude in order to account 

 fully for his results. The existence of light-pressure was deduced 

 from theoretical considerations, but it does not depend on theory 

 alone. The repulsion can be shown in the laboratory. Hull and 

 Nichols actually tried to make an artificial comet, using the fine 

 particles of lycopodium powder to show the repulsion of the tail. 

 Unfortunately, although a repulsive force was shown, it was due 

 mainly not to light-pressure, but to another effect. 



With a particle about tso^ofo i^^- i^i diameter, the repulsion is 20 

 times the attraction of gravity. Can we proceed to still smaller 

 particles and account for forces of :')6, 90 units or still higher ? It 

 appears not. The size mentioned is about the limit, and for small 

 particles, we find, instead of increasing repulsion, less repulsion. 

 Very minute particles offer practically no obstacle to the passage of 

 light which, instead of pressing against them, bends freely round. I 

 daresay by suitable assumptions, as to the density of the material, 

 forces of '^Q units might be accounted for, but the hypothesis of light- 

 pressure seems hardly competent to account for the greater repulsive 

 forces. It must not, hoAvever, be supposed that the theory of light- 

 pressure is thus discredited ; light-pressure must act, and probably 

 acts powerfully, on the minute particles which constitute a comet's 

 tail, but a careful analysis of the strange motions and transformations 

 taking place has convinced many astronomers that other forces are at 

 work modifying and in some cases increasing the repulsion. In this 

 connection the evidence that the repulsion is by no means a constant 

 force has obviously a most important bearing. 



At first it is with an almost overwhelming sense of the complexity 

 of the problem that we sit down before this mass of material, striving 



