SCIENTIFIC SUMMARY. 
89 
made, to test the accuracy of the method. In the case of the moon the 
motion in the line of sight is insensible, and the result of the observations 
is in very satisfactory agreement with this. The motion of Venus has been 
observed at both elongations, the comparison being made in some cases 
with the bright lines of hydrogen or magnesium, in others with the sky 
spectrum. The observations of the western elongation were made in the 
forenoon. The calculated motions have been deduced from the daily change 
of distance from the earth, as given in the u Nautical Almanac.” The 
observed motion is rather larger than the calculated, both for the approach 
and recession. Sir G. Airy remarks that this may be due in part to the 
disturbing effect of the juxtaposition of two nearly coincident lines $ but it 
seems not impossible that we have here an indication of the fact, already 
shown to be deducible from the observations of the recent transit, that the sun’s 
distance has been recently under-estimated. In Sir G. Airy’s paper the sun’s 
distance is taken as 91,260,000 miles, whereas the true distance appears to 
be about 92,000,000 miles. And of course our estimates of all the motions 
taking place within the solar system are affected in proportion to any error 
in the assumed distance of the sun. It is interesting to notice that we have 
in this spectroscopic method of measuring motions of recession and approach 
a means (which one day may prove exceedingly effective) of determining 
the dimensions of the solar system. 
Many attempts were made to measure the displacement of the F. line due to 
the rotation of Jupiter, but for a long time these were frustrated by atmo- 
spheric obstructions. On one occasion, however, the spectrum was seen fairly 
well, and measures obtained which give a result in remarkable accordance 
with the calculated value. u It is to be remarked,” says Sir G. Airy, “ that the 
method can be applied to Jupiter with peculiar advantage, as the equatorial 
velocity is very large, and its effect doubled by reflexion. Thus the observed 
displacement corresponds nearly to four times the equatorial velocity.” 
In measuring the rotation of the sun the great point aimed at was to pre- 
vent the sun’s heat from in any way affecting the position of the line 
observed, by expanding the slit unsymmetrically. The precautions adopted 
appear to have been sufficient, having, says Sir G. Airy, “ removed the 
sources of error to which results by former observers appear to have been 
exposed.” Particular care was also taken to avoid any bias from previous 
knowledge of the direction in which a displacement was to be looked for, 
the limb under observation not being determined till after the bisection had 
been made. u In the calculated motion,” says Sir G. Airy, u there is some 
uncertainty, as it is doubtful how far the period of rotation deduced from 
sunspots will apply to the chromatosphere, "to ^the rotation of which the 
observed displacement is due. There would appear to be no reason to sup- 
pose that it takes part in the general drift depending on the latitude, which 
has been remarked in sunspots.” These spectroscopic observations at 
Greenwich are on the whole very satisfactory. 
Spectra of the Planets. — M. Vogel has made researches upon the spectra of 
the planets, for which a prize has been awarded to him by the Copenhagen 
Academy. He finds the light of the planets in general to show the prin- 
cipal Fraunhofer lines. He considers the idea of a light proper to Jupiter 
and Saturn, as explaining their brightness, to be unfounded ; for u the 
