1918] on Gravitation and the Principle of Relativity 217 



tract and represent only half a yard. " But we saw that the pointer 

 did not change length when it turned." How did you tell that ? 

 What you perceived was an image of the pointer on the retina of 

 your eye, and you thought the image occupied the same space of 

 retina in both positions ; but your retina has also contracted in the 

 vertical direction without your knowing it, so that your estimates of 

 length in that direction are double what they should be. And 

 similarly with every test you could apply. If everything undergoes 

 the same change, it is just as though there were no change at all. 



We thus get a glimpse of what from our present point of view 

 must.be called the real world, strangely different from the world of 

 appearance. In the real world, .by changing position you extend 

 yourself like a telescope ; and the stoutest individual may regain 

 slimness of figure by an appropriate orientation. It must be some- 

 thing like what we see in a distorting mirror ; and you can almost 

 see a living-picture of this real world reflected in a polished door- 

 knob. 



If our speed through the ether happens not to be so great as we 

 have supposed, the contraction is smaller ; but it escapes notice in 

 our practical life, not because it is small, but because from its very 

 nature it is undetectable. And because this real world is undetect- 

 able we do not as a rule attempt to describe it. Not merely in every- 

 day life, but in scientific measurements also, we describe the world of 

 appearance. We do this by imagining natural objects to be placed, 

 not in the absolute space, but in a quite different framework of our 

 own contriving — a space which corresponds to appearance. In the 

 space of appearance a rod does not seem to change length when its 

 direction is altered ; and we use thab property to tjlock out our con- 

 ventional space, counting the length occupied by the standard yard- 

 measure as always a yard however its true length may vary. It is 

 found also that in like manner our time is a special time of our 

 own, different from the time we should adopt if our motion through 

 the ether were nil. Tliis is a perfectly right procedure ; it intro- 

 duces no scientific inexactness, and it is more in accordance with 

 the ordinary meaning attached to space and time ; the only thing 

 to remember is that this space and time framework is something 

 peculiar to us, defined by our mctiin, and it has not the meta- 

 physical property of absoluteness, which we have often unconsciously 

 attributed to it. 



Xow let us visit for a moment the star Arcturus, which is moving 

 relatively to us with a velocity of over 200 miles per second. Con- 

 sequently its motion through the ether is different from ours, and 

 the contraction of objects on it will be different. It follows that 

 our conventional space would not be suitable for Arcturus, because 

 it was specially chosen to eliminate our own contraction effects. 

 There is a different space and a different time proper to Arcturus. 

 We must then imagine each star carrying its own appropriate space 



