430 
Proceedings of the Royal Society 
the planet 100 years ago, was a star “11 Vulpeculae,” which Baily 
reported missing. The best orbit to suit the position of that star is — j 
n = 262° 
z=55°. 
If this be the true orbit the present position must be slightly altered. 
Those comets which have been in the solar system for an enor- 
mous time would have their orbits probably deflected, and would 
not therefore appear in these calculations. 
3. Note on the Velocity of Gaseous Particles at the Negative 
Pole of a Vacuum Tube. By Professor Tait. 
The recent exhibition by Professor G. Forbes of some of the latest 
of Mr Crookes’ experiments, together with what I had read or heard 
about their explanation, led me to infer that I might determine 
directly the velocity of the luminous particles near the negative pole 
(and perhaps at other parts) of a vacuum tube by means of obser- 
vations of the spectrum made in directions perpendicular to, and 
parallel to, the lines of motion of the incandescent particles of gas. 
I made the attempt on some charcoal-bromine vacuum tubes, for 
which I have to thank Professor Dewar, but I found the light to be 
so feeble that it was impossible to employ an eight-prism spectro- 
scope. A one-prism spectroscope, when the j spectra taken in and 
perpendicular to the direction of motion of the particles were placed 
side by side, showed merely that the velocity could not amount to 
anything like 90 miles per second. There did seem to he a very 
slight shifting of the former spectrum towards the violet, but this 
appearance was probably due to the fact that its light had been 
weakened by two reflections, while that of the other was taken 
direct. 
It was evident that one cause at least of the failure is the great 
loss of light by multiplied reflections when a powerful spectroscope 
is employed. Thus I was driven to try the only other available 
method with which I was acquainted, and which indeed I had 
employed for more than ten years as an occasional part of the routine 
work in the physical laboratory. This method depends upon rotation 
(by quartz) of the plane of polarisation, combined (when necessary)' 
