246 The N.Z. Journal of Science and Technology. [Jan. 
be due to a gravitational field would be perceived by him in exactly the 
same way if the gravitational field were not present, provided that the 
observer makes his system of reference move with the acceleration which 
was characteristic of the gravitation at his point of observation. This, in 
effect, is saying what I tried in other words to say. On the projectile 
the gravitation was absent, but on the earth it was present ; but to the 
inhabitants of the projectile it was the earth which appeared to be altering 
its speed, for 40 them the projectile was at rest. There is no possibility of 
distinguishing between them. 
Now, building on this foundation, Einstein and others have worked out 
three possible checks upon this theory—three things which ought to happen 
if the ideas which lie at the bottom of it are correct. For many years 
astronomers have known that Mercury behaved somewhat—or, rather, 
quite—unaccountably, supposing him to be under forces which were exactly 
expressed by Newton’s law. All the planets near the sun—viz., Mercury, 
Venus, the earth, and Mars—alter their position of perihelion as time goes 
on ; but whereas that motion in the case of Venus, the earth, and Mars 
was quite well accounted for on the older ideas, there was a large discordance 
which was quite unaccounted-for in the case of Mercury ; indeed, this dis¬ 
cordance amounted to as much as 42 seconds of arc per century—a quite 
enormous amount. A first check, then, on Einstein’s views was got when, 
applying the modified law of gravitation which was derived by him to 
Mercury’s motion, these 42 seconds exactly disappeared, without at the 
same time making the agreement in the case of Venus, the earth, and Mars 
any worse ; indeed, the agreement between the new theoretical deductions 
and observed facts in the case of these three planets was distinctly im¬ 
proved as compared with the older theory, and in the case of Mercury it 
was enormously improved. This, shortly, is check 1. 
Check 2 was obtained last year at an eclipse of the sun, and it is that 
check which has caused so much attention to be given—even in such 
popular newspapers as the Educational Supplement to the Times — to 
Einstein’s theory, and it is that check which causes me to be standing 
here to-night, struggling to make clear to you—a thing that is not by any 
means clear to me—what all the fuss is about. It turns out that if 
Einstein’s ideas are correct all moving energy should be subject to gravi¬ 
tational influences, and that light and all other forms of moving energy 
should be deflected from its path on passing through a gravitational field, 
just as a comet or other material body moving with the same velocity would 
be deflected. Now, during an eclipse of the sun we can see stars the path 
of whose light from the star to the earth passes close to the edge of the sun, 
and hence through an intense gravitational field. If, then, the light from 
that star is deflected, the star would not appear to be in quite the same 
position relatively to other stars whose light passed to us by a path which 
was not so close to the sun’s edge as that of the one we are considering. 
Thus by measuring the positions of the stars in that exact part of the 
heavens in which the sun would be at the moment of an eclipse—in measur¬ 
ing the positions of these stars, I say, during the eclipse, and also some 
months before and after it, when the sun had got well away again, there 
should be some shift in the positions of some and not in those of others. 
Now, on either the older or the newer theory of gravitation a shift should 
take place, but on the older one it should be half what it would be on the 
newer. In figures, on Einstein’s theory the shift of position should be 
1-74 seconds of arc, and on the older theory it should be 0-87 seconds. 
It was thus this small amount of extra shift that the astronomers went 
