Explanatory Notes 123 



before it was geodetically observed by accurate surveying. 

 See, for instance, Pioneers of Science, previously referred 

 to. 



Page 54 



The pressure exerted by light was predicted by Clerk 

 Maxwell, and by Bartoli, and first observed by Nicholls 

 and Hull. The consequent or associated momentum 

 conveyed by a wave front, and a number of the effects 

 deducible from this process, have been worked out most 

 instructively by Professor J. H. Poynting, who has pub- 

 lished a little book on the subject called The Pressure of 

 Light (S.P.C.K.). 



Pages 62 and 63 



The Michelson-Morley experiment detects no difference 

 between a journey to and fro and a journey right and left, 

 if one is facing the hypothetical ether stream presumably 

 caused relatively to the earth by its rapid motion through 

 space, and if one is sending a beam of light in these direc- 

 tions by means of mirrors mounted on some rigid body, 

 such as a block of stone or metal. (See also Note to pp. 

 43-64.) But elementary arithmetic shows that it ought 

 to take slightly longer, going up and down the stream, 

 than there and back across it. Hence, either there is no 

 such ether stream, or else the block on which the mirrors 

 and other optical appliances are mounted changes, so as to 

 be shorter by a compensating amount in the direction of 

 motion as compared with its breadth in the transverse 

 direction. The required change is exceedingly small, 

 equivalent to a shrinkage of three inches in 8000 miles, 

 yet on the electric theory of matter something of this mag- 

 nitude almost certainly ought to occur. This is the famous 

 FitzGerald-Lorentz hypothesis, referred to in the text. 

 (See pp. 53 and 64; also my book Electrons, chapter 

 xvi., about cohesion.) 



