On the Nature of the Light emitted by heated Tourmaline. 891 
ence to the moon’s declination for any given period of the year, con- 
sists chiefly in the difference of the relative values of the maxima and 
minima, the differences of epochs being small. Thus for perihelion, 
the moon furthest north, the principal maximum occurs at the infe- 
rior passage ; the moon on the equator going south, the two maxima 
are nearly equal; the moon furthest south, the maximum at the 
superior passage is by far the greatest: on the equator going north, 
the two maxima are again nearly equal; and so on for other epochs. 
10th. The moon’s action is chiefly, if not wholly, dependent on 
the position of the sun, or (which is the same thing) on the position 
of the earth relatively to the sun; and the law of the lunar action at 
the magnetic equator resembles in some points that for the solar 
action at the same epochs. ‘Thus about aphelion there is a minimum. 
of easterly (maximum of westerly) declination produced by the Junar 
action, as well as by the solar action, for these two bodies near the 
superior meridian; whereas about perihelion both actions for the 
sun and moon near the superior meridian produce maxima of easterly 
declination. A like analogy holds for near the epochs of sun- 
rise and moonrise. 
June 14.—General Sabine, R.A., Treasurer and Vice-President, in’ 
the Chair. : 
The following communication was read :— 
“On the Nature of the Light emitted by heated Tourmaline.” By 
Balfour Stewart, Esq., M.A. : 
Some months ago I had the honour of submitting to the Royal, 
Society a paper on the light radiated by heated bodies, in which it 
was endeavoured to explain the facts recorded by an extension of 
the theory of exchanges. 
Having mentioned the difficulty which I had in maintaining the 
various transparent substances at a nearly steady red heat for a 
sufficient length of time in experiments demanding a. dark back- 
ground, Professor Stokes suggested an apparatus by means of which 
this difficulty might be overcome; and it is owing to his kindness in 
doing so that I have been enabled to lay these results before the 
Society. 
The apparatus consists of a thick, spherical, cast-iron bomb, about 
5 inches in external and 3 inches in internal diameter—the thickness 
of the shell being therefore 1 inch. It has a cover removeable at 
pleasure. There is a small stand in the inside, upon which the sub- 
stance under examination is placed, and when so placed it is pre- 
cisely at the centre of the bomb. Two small round holes, opposite 
to one another, viz. at the two extremities of a diameter, are bored in 
the substance of the shell. If, therefore, the substance placed upon 
the stand be transparent, and have parallel surfaces, by placing these 
surfaces so as to front the holes, we are enabled to see through the 
substance, and consequently through the bomb. Let the bomb with 
the substance on the stand be heated to a good red heat, and then 
withdrawn from the fire and allowed to cool. It is evident that the 
eooling of the substance on the stand will proceed very slowly, as it 
