On Light. 13 
of the large pyramids in Egypt might profitably be employed, in- 
stead of a mountain, for this experiment. Such a body offers se- 
veral advantages for the purpose. In the first place, the mass is 
sufficiently large, standing on a base of about the size of the whole 
space of Lincoln’s Inn Fields, and of a height almost double of 
that of St. Paul’s steeple, or near three times the height of the 
Monument: then the station for the plummet, or zenith sector, 
could be taken much nearer the centre of the mass than on amoun- 
tain, which would give a larger quantity of deviation of the plum- 
met; then the regular figure and the known composition of the 
mass would yield great facilities in the calculation ofits attraction. 
Lastly, the deviation of the plummet might be observed on all 
the four sides. Should such a project take place, it wil! be best 
to take the stations at about one-fourth of its altitude above the 
base, that being the place where the deviation of the plummet 
would be the greatest. Finally, so favourable for such an ex- 
periment do those circumstances appear, and so anxious are my 
wishes for its completion and success; that, were it not for my 
great age and little health, I should be glad to make one in any 
party to undertake so interesting an expedition. 
Bedford Row, March 17, 182). Cuar.es Hutron. 
Il. On Light. By Anprew Ure, M.D. Professor of the 
Andersonian Institution, Glasgow. 
{Concluded from vol. lvii. p. 418.] 
Ill. Polarization of Light. 
Tus new branch of optical science, sprung from the ingenuity 
of Malus. It has been since cultivated chiefly by M. Biot in 
France, and by Dr. Brewster in this kingdom. I am happy to 
observe, that Mr. Herschel has lately entered the lists under very 
favouralile auspices. 
If asolar ray fall on the anterior surface of an unsilvered mirror 
plate, making an angle with it of 35° 25’, the ray will be reflected 
in a right line, so that the angle of reflection will be equal to the 
angle of incidence. In any point of its reflected path, receive 
it on another plane of similar glass, it will suffer in general a se- 
cond partial reflection. But this reflection will vanish, or become 
null, if the second plate of glass form an angle of 35° 25’ with 
the first reflected ray, and at the same time be turned, so that 
the second reflection is made in a plane perpendicular to that in 
which the first reflection takes place. For the sake of illustration, 
suppose that the plane of incidence of the ray on the first glass, 
coincides 
