515 
for the observed effect, but not the same number of photos have been 
taken on all plates as on Nr. 48. The velocities used ranged between 
750 and 950 em./see. the length of the quartz column being 100 em. 
for nine plates, and 140 em. for two. The obtained values may now 
be used in two ways to derive a final result from them. For each 
plate a mean value can be derived, and the arithmetical mean may 
be taken of the eleven values thus obtained. In this way: 
0,146 += 0,012, 
as final result of the effect reduced to a velocity of 1000 cm./sec. and 
a length of quartz of 100 cm., the mean error being recorded after 
the + sign. 
Another way in which the values can be combined is by taking 
the arithmetical mean of the fifty-one values. Thus we find: 
0,148 + 0,006. 
From formula (4), which was given in our communication I, and 
will be proved presently, follows for the theoretical value of the 
effect : 
0,143 
6. With red light 2 — 6510 A. U. twenty-seven values have been 
obtained for the effect on six plates. To eight of them corresponds a 
quartz column of 140 em., to nineteen one of 100 cm. The velocities 
range between 750 and 960 em./sec. 
The result, when the mean values of the different plates are 
combined, is: 
0,123 + 0.014. 
The arithmetical mean of the twenty-seven separate values yields: 
0,125 + 0.007. 
The calculation gives for the expected effect: 
0.115. 
7. The results with violet light 2 == 4750 A. U. should be received 
with some diffidence, as it appeared afterwards that the violet filter 
transmitted some red light, which had not been detected at first. 
Hence it is possible that this cause slightly vitiated the later series. 
It must be said, however, that no trace of change could be ascer- 
tained in the values of the later series. 
