'') again leads to an equation of transformation of the same form as that of the initial equations. The parameter
'' that occurs in the new equation depends in a definite way on
' and
.
2. The contractions of the lengths depend only on the value of the parameter
. We must, of course, from the very outset reckon with the possibility that the length of a rod measured in the system that is at rest comes out differently when measured in the moving system. Now, condition 2 requires that if contractions occur (that is, changes of length in these various methods of determination) values are to depend only on the magnitude of the velocity of both systems and not on the direction of their motion in space. Thus this postulate endows space with the property of isotropy, and is in fair correspondence with the postulate of [section 3a], which states that it must be possible to compare each line-element with every other in length independently of its position in space, and its direction.