18,4 Perkins: Absolute Units and Relativity Principle 347 
SUMMARY 
1. It is pointed out that the velocity of light is only one of 
many physical quantities, equally electromagnetic in nature, 
which are apparently not affected by the motion of the earth 
through space. 
2. In view of this fact it is a fallacy to assume any one of 
these quantities to be a Universal Constant and to deduce 
from this assumption that the others are not real constants 
but only apparent constants. 
3. Whatever view we take of the situation we can define our 
units of length, time, and mass in such a way as to make them 
independent of the velocity through space of the system con- 
sidered, and it follows that absolute simultaneity is definable. 
