46 TIME 



How is it that, starting from the same Seen-Now 

 lines, you do not reproduce my Now line? It is because 

 a certain measured quantity, viz. the velocity of light, 

 has to be employed in the calculations; and naturally 

 you trust to your measures of it as I trust to mine. 

 Since our instruments are affected by different Fitz- 

 Gerald contractions, etc., there is plenty of room for 

 divergence. Most surprisingly we both find the same 

 velocity of light, 299,796 kilometres per second. But 

 this apparent agreement is really a disagreement; be- 

 cause you take this to be the velocity relative to your 

 planet and I take it to be the velocity relative to mine.* 

 Therefore our calculations are not in accord, and your 

 Now line differs from mine. 



If we believe our world-wide instants or Now lines 

 to be something inherent in the world outside us, we 

 shall quarrel frightfully. To my mind it is ridiculous 

 that you should take events on the right of the picture 

 which have not -happened yet and events on the left 

 which are already past and call the combination an 

 instantaneous condition of the universe. You are 

 equally scornful of my grouping. We can never agree. 

 Certainly it looks from the picture as though my 

 instants were more natural than yours; but that is 

 because / drew the picture. You, of course, would 

 redraw it with your Now lines at right angles to your- 

 self. 



* The measured velocity of light is the average to-and-fro velocity. 

 The velocity in one direction singly cannot be measured until after the 

 Now lines have been laid down and therefore cannot be used in laying 

 down the Now lines. Thus there is a deadlock in drawing the Now lines 

 which can only be removed by an arbitrary assumption or convention. 

 The convention actually adopted is that (relative to the observer) the 

 velocities of light in the two opposite directions are equal. The resulting 

 Now lines must therefore be regarded as equally conventional. 



