22(5 Professor A. S. Eddington [Feb. 1, 



and therefore must have mass — ^a coiickision which has ])een found 

 true experiraeiitally, because light falhng on any object exerts a 

 pressure just as a jet of water would. We ordinarily measure mass in 

 pounds, and it is quite proper to speak of " a pound of light," just as 

 we speak of a pound of tobacco. In case anyone should be thinking 

 of s'oing to an Electric Light Company to buy a pound of light, I 

 had better warn you that it is a rather expensive commodity. They 

 usually prefer to sell it by a mysterious measure of their own, called 

 the Board of Trade Unit, and charge, at least, threepence a unit. At 

 that rate I calculate that they would let you have a pound of light for 

 £141,615,000. Fortunately, we get most of our light free of charge, 

 and tlie sun showers down on the earth 160 tons daily. It is just as 

 well we are not asked to pay for it. 



But although light has mass, it does not follow that light has 

 weight. Ordinarily, mass and weight are associated in a constant 

 proportion, but whether this is so in the case of light can only be 

 settled by experiment — by weighing light. It seems that it should 

 be just possible to do this. If a beam of light passes an object which 

 exerts a gravitational attraction, then, if it really has weight, it 

 mutt drop a little towards the object. Its path will be bent just as 

 the trajectory of a rifle bullet is curved owing to the weight of the 

 bullet. The velocity of light is so great that there is only one body 

 in the solar system powerful enough to make an appreciable bend in 

 its path, namely, the sun. If we could see a star close up to the edge 

 of the sun, a ray of light coming from the star would l)end under its 

 own weight, and the star would be seen slightly displaced from its 

 true position. During a total eclipse stars have occasionally been 

 photographed fairly close to the sun, and with care it should be 

 possible to observe this effect. There is a magnificent opportunity 

 next year when a total eclipse of the sun takes place right in the 

 midst of a field of bright stars. This is the best opportunity for 

 some generations, and it is hoped to send out expeditions to the line 

 of totality to weigh light according to this method. 



In any case great interest must attach to an attempt to settle 

 whether or not light has weight. But there is an additional impor- 

 tance, because it can be made a means of confirming or disproving 

 Einstein's theory. On Einstein's theory light must certainly have 

 weight, because mass and weight are viewed by it as two aspects of 

 the same thing ; but his theory predicts a deflection twice as great 

 as we sliould otherwise expect. Apart from surprises, there seem to 

 be tliree possible results : (1) a deflection amounting to 1*75" at the 

 limb of the sun, which would confirm Einstein's theory ; (2) a de- 

 flection of 0*83" at the limb of the sun, which would overthrow 

 Einstein's theory, but would estaljlish that light was subject to 

 gravity ; (3) no deflection, which would sliow that light though 

 possessing mass has no weight, and hence that Newton's law of pro- 

 portionality between niass and gravitation lias broken down in 

 another unexpected direction. 



