224 
lis the distance over which the movable mirror has been displaced 
from out its new position. We find d for one definite wavelength 
du du 
out of — K d=—tiord x(— À x) == tÀand then d’ out of (1). 
dà d 
Perhaps we can replace the unmovable mirror in the interfero- 
meter of MicneLsoN by a concave one '). 
In conclusion I wish to express my cordial thanks to Prof. 
Ornstein and Prof. Juttus for their suggestions and the interest shown 
in this research. The research which Prof. Ornstein originally asked 
me to undertake, was the first step on the road by which | have 
ultimately sueceeded in finding the expedient which was to prove 
able to solve the problem theoretically in a perfectly satisfactory way. 
I have followed the logical train of reasoning which has led to 
this, also in this communication, because through the considerations on 
the four groups of points P, P’, P", PY it could then be naturally 
proved as second thesis that the required distribution of energy 
cannot be determined by any of the current instruments, in whatever 
way the observations made by them should be combined. The phase- 
shifting, independent of the wavelength and differing from 2, was 
accordingly not only a possible, but also a necessary expedient to 
accomplish the task we had set ourselves. 
Utrecht, May 1919. Institute for Theoretical Physics. 
1) Govy, Ann. de Chimie et de Physique (6) 24, pag. 198, 1891. 
