248 
SIR J. CONROY ON THE AMOUNT OF LIGHT REFLECTED 
Part I. 
Section I.—Determination of the Amount of Light Transmitted. 
A photometric arrangement similar to the one described in Pickering’s ‘ Physical 
Manipulations,’ vol. 1, p. 132, was used for the greater number of these determina¬ 
tions (see Plate 8, fig. 1). Two similar pieces of looking glass 125 mm. X 125 mm. 
were fixed vertically at the ends of a horizontal board rather more than 2 metres long 
and 27 cm. wide, the mirrors being 2 metres apart; a small Argand gas burner, giving 
a flame 15 mm. in diameter, was fixed opposite the middle point of the line joining 
the centres of the mirrors, and at a horizontal distance of 20 cm. from the line; the 
mirrors were so adjusted that they reflected the light of the lamp towards each other. 
A block of wood, resting on four small metal rollers and guided by two strips of 
wood fixed 7‘8 cm. apart on the upper surface of the horizontal board, carried the 
photometer; an index was fixed to the wood block, and a scale, divided into milli¬ 
metres, to the board, the zero being at one of the mirrors. 
The photometer of which a description was given in ‘ Poy. Soc. Proc.,’ vol. 35, 
p. 27, and ‘Phil. Mag.,’ series 5, vol. 15, p. 423, was used (Plate 8, fig. 1a); it con¬ 
sisted essentially of two pieces of white paper so placed that, whilst each was 
illuminated by one only of the two lights to be compared, both were visible to the 
observer. Three pieces of wood were screwed to the block, and between these the 
photometer was placed ; this arrangement, whilst permitting the photometer to be 
reversed, so that the light from each of the two mmrors could be made to fall first on 
the one and then on the other paper, ensured its always being replaced in the same 
position. 
The whole arrangement was optically equivalent to two exactly similar sources of 
light which always retained the same relative intensity, and were at a distance apart 
of a little more than twice the distance between the two mirrors. 
A wooden screen was placed between the lamp and the observer, and, in order to 
cut off stray light reflected from the horizontal board, screens, with apertures in 
them rather larger than the apertures in the photometer box, were placed on either 
side of it, and between it and the mirrors; the edges of the apertures in the screens 
being formed of metal filed to a feather edge, and then, together with all the wood 
work, painted a dead black. 
Assuming that the two sides of the Argand flame were equally bright, that the 
reflective powers of the two looking glasses were equal, and that there was no stray 
light, or at least that there was an equal amount from either side, then at a point 
midway between the two mirrors the intensities of the light reflected from the mirrors 
’would necessarily have been equal, and, in order to determine the amount of light 
which passed through any transparent substance, it would merely have been necessary 
