of Edinburgh , Session 1866-67. 
167 
9. On Yortex Motion. By Professor Sir William Thomson . 
10. Note on the Radiant Spectrum. By Professor Tait. 
This was a preliminary notice of a set of experiments, then in 
progress under very unfavourable circumstances, on the rare occa- 
sions when a ray of sunlight was procurable. The production of 
the phenomenon was traced to the peculiar texture of the membrane 
covering the cornea. The dependence of the whole on parallax 
was confirmed by the appearance of the radiant spectrum in precisely 
the same position with reference to the bright spectrum, when it 
was produced artificially by interposing a plate of slightly ground 
glass in the course of the light, so long as this plate was sufficiently 
near the eye to produce the phenomenon at all. A farther confir- 
mation was given by the fact that, if the eye be near the prism, 
the appearance is the same on whichever side of the prism the 
ground glass be placed. The effects of achromatic prisms, of 
absorption by coloured glasses, and of the use of various kinds of 
homogeneous light taken from a pure spectrum, were also described. 
11. Note upon a Method of varying Weights by Minute 
Quantities. By J. A. Broun, F.R.S. 
A notice of a Gravimeter, proposed by me, appeared in the Pro- 
ceedings of the Royal Society of Edinburgh early in 1861. The 
instrument, with various modifications, was forwarded to me in 
India about three years thereafter, and was found to have several 
imperfections which could have been corrected only by my own 
supervision of the work as it proceeded. His Highness the Rajah 
of Travancore has been good enough to sanction a sum of money 
for the construction of a second instrument, with all the precautions 
experience has suggested. 
There are two methods by which the observations may be made. 
One, by which there is a constant angle of torsion of a single wire 
giving a variable angular movement to the weight suspended by 
two wires (depending on the force of gravity at the place of obser- 
vation). The other, by which the weight suspended is varied, and 
