463 
formula (1). Micnetson and Moruey’s investigation of 1886, performed 
with MicHeLsoN'’s interferometer in one of the numerous forms into 
which as a real Proteus this wonderful instrument is capable of being 
changed, gave with white light a value of the convection coefficient 
which was in excellent agreement with the coefficient that follows 
for yellow sodium light from formula (1). 
Experiments that have been carried out by me with different 
colours ranging from violet to red, and in which the axial velocity 
of water in the tube was directly measured have been communi- 
cated by me in different papers to this Academy *). The validity of 
the formula (2) with the term of dispersion could be demonstrated 
with an accuracy exceeding 0.5 °/,. The optical effect that is measured 
in these experiments, is a displacement of interference bands, which 
is given in parts of the distance of two bands by the formula: 
AN (ome mende. 4 
=a ow aa uw, ° . 5 ° 5 . (3) 
in which / represents the length of the whole liquid column which 
is in motion. 
2. The apparatus that has been used for the investigation of the 
motion of light in solid substances, is shown diagrammatically in 
Fig. 1. 
fig. 1 on a scale of '/,,, and might also be used with simple modi- 
fications for the investigation of costly liquids and compressed gases. 
The moving, transparent substance is rigidly connected with a 
piece DE, and can therefore rapidly move to and fro parallel to 
1) Zeeman, These Proc. 17, 445, 1914; 18, 398, 1915; 18, 711, 1915; 18, 1240, 
1916; 19, 125, 1916. 
