892 PEOFESSOE BUN'SEN AKD DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 
appears black upon a white ground. In order to obtain the wished-for proof, the flame 
{g) is first so placed that- the rays fall perpendicularly on the paper screen (a), and then 
moved over the graduated circle (B), so that the rays fall upon the screen with the 
angle of incidence (p, care being in each case taken that the lamp is moved along the 
scale (C) until the spot of stearine disappears, so that in both positions of the flame the 
illumination of the side of the screen [e) next the eyepiece should be the same. "SMien 
the distances have been found at which the flame {g) must be placed in the two positions, 
in order that the hack of the paper screen should be illuminated to the same extent, it is 
easy to calculate the amount of light which falls on the front of the screen, and which, 
in order to answer the required purpose, ought to be equal to that seen through the 
tube (5). If <p and p, represent the complements of the two angles of incidence, mea- 
sured on the divided quadrant, with which the light fell upon the paper screen, and r r^ 
the distances between the screen and flame at which the stearine spot vanished, then 
the following equation is true, provided the assumptions made at the commencement 
are correct sin p . rf ^ 
sin 
Experiment gave the follomng numbers ;■ — 
Table IV. 
Writing-paper. 
Drawing- 
paper. 
Filter- 
paper. 
p... ......... 
90° 
90° 
90° 
o 
90° 
r 
250 
262 
203 
146 
300 
Pi ......... 
56° 15' 
33° 45' 
22° 30' 
33° 45' 
33° 45' 
......... 
205 
167 
81 
103 
186 
sin p.rj 
sin 
0-83 
0*73 
0*42 
0-89 
0-69 
It is seen that the numbers in the lowest horizontal column difier mdely from 1, and 
that no simple relation exists between the amount of light falling on the front and that 
issuing from the back of the screen. It now only remained to be seen whether a better 
result is obtained when the diaphragm with the stearine spot is itself directly illuminated. 
For this purpose the experiments were repeated, with the sole difference that the dia- 
phragm e was placed at the end of the tube at a instead of the paper screen. With 
this arrangement the following numbers were obtained : — 
Table V. 
P 
90° 
90° 
90° 
90° 
90° 
r 
225 
199 
178 
220 
224 
Pi 
22° 30' 
33° 45' 
45° 0' 
67° 30' 
O 
GO 
7-1 ......... 
140 
150 
150 
214 
221 
sin p.rj 
1-01 
1-02 
1-00 
1-02 
0-993 
sin 
The numbers in the lowest division are all so nearly equal to 1, that the slight dif- 
ferences may with certainty be ascribed to the unavoidable errors of observation. 
