MECHANICS AND OPTICS. 219 



explain so long as we neglect the square of aberration, 

 and for a long time experiments were not sufficiently 

 accurate to make it necessary to take this into account. 

 But one day Michelson thought out a much more 

 delicate process. He introduced rays that had 

 traversed different distances after being reflected by 

 mirrors. Each of the distances being about a yard, 

 and the fringes of interference making it possible to 

 detect differences of a fraction of a millionth of a 

 millimeter (^-g-WzTDiyoth of an inch), the square of 

 aberration could no longer be neglected, and yet the 

 results were still negative. Accordingly, the theory 

 required to be completed, and this has been done by 

 the hypothesis of Loretits and Fitz-Gerald. 



These two physicists assume that all bodies in- 

 volved in a transposition undergo a contraction in the 

 direction of this transposition, while their dimensions 

 perpendicular to the transposition remain invariable. 

 This cojttraction is the same for all bodies. It is, more- 

 over, very slight, about one part in two hundred million 

 for a velocity such as that of the Earth. Moreover, 

 our measuring instruments could not disclose it, even 

 though they were very much more accurate, since 

 indeed the yard-measures with which we measure 

 undergo the same contraction as the objects to be 

 measured. If a body fits exactly to a measure when 

 the body, and consequently the measure, are turned in 

 the direction of the Earth's motion, it will not cease to 

 fit exactly to the measure when turned in another 

 direction, in spite of the fact that the body and the 

 measure have changed their length in changing their 

 direction, precisely because the change is the same for 

 both. But it is not so if we measure a distance, no 



