288 
JDK. J. YOUNG AND PROFESSOR G. FORBES ON THE 
Usual source of light. 
V elocity. 
Michelson .... 
Sun near horizon. . 
299,940 
CoENU . 
Lime light .... 
300,400 
Young and Foebes . 
Electric light . 
301,382 
In every case the more refrangible the mean colour of the source of light the greater 
is the velocity. 
In Michelson’s observations the image of the slit was described as indistinct 
and covering a sensible space. From our results it would appear that the width of his 
spectrum between mean red and blue would be about 2 millims. But it would be a 
very impure spectrum, and it is only by employing absorptive media, or part of a pure 
spectrum, to give colour to the light used, that we should expect him to detect the 
difference. 
Solar eclipses and occultations. —We might expect that the sun or a star on dis¬ 
appearing behind the moon would appear to be red, and on reappearing would first 
flash out blue. But light takes only L-|- second to come from the moon and the one- 
fiftieth or one-hundredth part of this is too small an interval of time for us to have 
any hopes in this direction. 
Eclipses of Jupiter's satellites. —There must be a difference in time between the 
disappearance or reappearance of the blue or red parts of the light of a satellite, 
amounting to about half a minute. But the change is so gradual and the light when 
near eclipse so feeble that we fear this would be a very difficult determination. 
AbeiTation, —A star exposed fully to the action of aberration must be drawn out 
into a spectrum parallel to the direction of the earth’s motion. The length of the 
spectrum between the mean red and blue must be about G^'36. It is possible that 
Avith a reflector of good definition this effect might be detected. 
Temporary stars. —The mean light from the nearest star whose parallax is known 
takes 31 years to reach the earth. The difference in time taken by the blue and red 
rays to reach us must be 13 days. The star T Corona flashed out suddenly in 1866. 
It was first seen on May 9, and there seems to be good evidence to show that it was 
not conspicuous a weak earlier. On May 9 it was of the second magnitude, and it 
diminished in brightness for some days at the rate of a magnitude a day. On the 
12th May it was examined by Dr. Huggins with a spectroscope and showed bright 
lines in very different parts of the spectrum. This is somewhat contrary to what we 
should have expected. But Ave are not aware that the evidence for the identity of 
the temporary star and the small one known to be in that position is irresistible, nor 
do we know that the parallax of that small star has been very carefully studied, nor is 
the evidence as to the invisibility of the star at the beginning of the month conclusive. 
Vendable stars .— When a variable star brightens the first colour to reach us in 
increased intensity should be blue, and when it fades the last colour to cease shining 
