1868. } Luminous Intensity of Light. 363 
ing surface (white paper or finely-ground porcelain), so that when 
DC is pushed into the end B, one white surface D B may be 
illuminated (as in Fig. 2) by the candle, and the other surface B C 
by the lamp. If the eye-piece A is removed, the observer, looking 
down the tube G B, will see at the end a luminous white disc divided 
into two parts, one half being illuminated by the candle H, and the 
other half by the lamp F. By moving the candle E, for instance, 
along the scale, the illumination of the half D B can be varied at 
will, the illumination of the other half remaining stationary. 
The eye-piece A (shown enlarged at Fig. 3) will be understood 
by reference to Fig. 1, the same letters 
representing similar parts. At L is a 
lens to collect the rays from D BC 
(Fig. 2), and throw the image into the 
proper part of the tube. At M is 
another lens, so adjusted as to give a 
sharp image of the two discs into which 
I is divided by the prism K. ‘The part 
N is an adaptation of Arago’s polarimeter, 
which consists of a series of thin plates 
of glass capable of moving round the 
axis of the tube, and furnished with a 
pointer and graduated arc (shown at 
A, G, Fig. 2), By means of this pile it 
is possible to partially polarize the rays 
coming from the illuminated discs in 
one or the other direction, and thus bring 
to the neutral state the partially polarized 
beam ¢ d, (Fig. 1,) so as to get the 
images g r free from colour. It is so 
placed that when at the zero point it 
produces an equal effect on both discs. 
The action of the instrument is as 
follows. The standard lamp being placed 
on one of the supporting pillars which 
slide along the graduated stem (Fig. 2), 
it is adjusted to the proper height, and 
moved along the bar to a convenient 
distance, depending on the intensity of the light to be measured: 
the whole length being a little over four feet, each light can be 
placed at a distance of twenty-four inches from the disc. The flame 
is then sheltered from currents of air by black screens placed round, 
and the light to be compared is fixed in a similar way on the other 
side of the instrument. The whole should be placed in a dark 
room, or surrounded with non-reflecting screens; and the eye must 
also be protected from direct rays from the two lights. On looking 
Fia. 3. 
