260 
SIR J. CONROY ON THE AMOUNT OF LIGHT REFLECTED 
15 mm. plate. 
OL. 
Pt-rcentage amount 
of light transmitted. 
Witliout glass. 
AVith glass. 
O / 
O > 
4i 54 
45 29 
88-73 
40 23 
43 42 
90-08 
40 17 
43 44 
89-82 
Mean 89-54 
This method is clearly incapable of giving very accurate results. It is difficult to 
judge of the equality of the illumination in the two halves of the field, and also the 
angle through which the Nicol has to be turned to make the comparison is small, and, 
therefore, a slight error in the determination of the value of a makes a very, 
considerable one in the result. 
The measurements with the polarising photometer not being entirely satisfactory, 
another form of photometer was devised (Plate 8, figs. 3 and 3a). It consisted 
essentially of two white surfaces illuminated by the same lamp, the light falling very 
nearly perpendicularly on both. One surface was at a constant distance from the 
lamp, whilst the other could be brought nearer to, and moved further from it ; one^ 
surface was seen directly, and the other through the glass to be examined, and then 
the distance of the movable surface from the lamp altered till both surfaces appeared 
equally bright. 
One surface was fixed at a distance of 78 cm. from the lamp and in the same 
horizontal plane, whilst the second was fixed to a vertical screen, which could be 
moved backwards and forwards by means of a pulley and catgut band along a, board 
with a divided scale. The surfaces consisted of a double thickness of white paper, as 
it was found by taking a double thickness the apparent illumination was increased, a 
portion of the light which passed through the first paper being reflected back by the 
second. In front of the lamp two right-angled prisms were placed. They were held 
in position by two pieces of wood, through which a screw was passed, one prism being 
slightly in advance of the other, and overlapping it to a small extent. To adjust the 
prisms they were placed on a smooth table, the one resting directly on the table, and 
the other on a thin piece of card, and, after being adjusted, fixed in position by means 
of the screw. When so placed, the line dividing the two fields of view was much 
narrower than when the front surfaces of the prisms were in the same vertical plane 
and their edges in contact. A screen was placed between the prisms and the lamp, 
which were only 8 cm. apart. 
The light (A the lamp fell nearly perpendicularly on the white paper, and the 
