400 PEOFESSOE BUIS'SEN AKD DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 
Series of Experiments XIII. 
Experiment 1. 
Experiment 2. 
Experiment 3. 1 
T. 
ru 
t. 
B. 
Bo— Bj 
T. 
n. 
h* 
t. 
B. 
Bo—Bj j 
T. 
n. 
tj. 
t. 
B. 
Bp— Bj 
Bo 
Bo 
Bo 
0 
2 
30-2 
53-4 
0-10893 
0-00000 
0 
2 
10-9 
421 
0-10424 
0-00000 
0 
1 
5'0 
42-6 
0-05347 
0-00000 
2 
1 
17-2 
92-6 
0-10722 
0-01551 
17 
2 
45-0 
59-0 
0-10381 
0-00413 
17 
1 
22-5 
59-0 
0-05190 
0-02935 
18 
2 
29-7 
52-3 
0-10651 
0-02222 
27 
2 
30-1 
50-0 
0-099403 
0-04643 
27 
1 
18-3 
50-0 
0-04508 
0-15695 
20 
2 
19-1 
46-7 
0-10566 
0-03002 
102-5 
1 
33-3 
69-1 
0-05091 
0-51164 
76 
1 
18-4 
35-5 
0-02432 
0-54517 
26 
2 
15-0 
44-0 
0-10381 
0-04700 
125-0 
1 
29-1 
60-7 
0-04493 
0-56901 
102-5 
1 
63-4 
69-1 
0-00811 
0-84833 
42-5 
2 
40-9 
56-5 
0-10253 
0-05854 
150-0 
1 
35-8 
61-2 
0-03612 
0-65354 
125 
1 
58-3 
60-7 
0-00341 
0-93623 
65-5 
2 
39-7 
45-6 
0-07323 
0-32773 
198-0 
1 
52-4 
71-2 
0-02673 
0-47362 
150 
1 
60-1 
60-1 
0-00000 
88-5 
1 
16-4 
59-9 
0-06185 
0-43121 
136-5 
1 
35-5 
68-6 
0-04707 
0-56789 
i 
163-0 
1 
50-5 
79-5 
0-04124 
0-62135 
! 
The curves Nos. 1, 2 and 3, fig. VII., Plate XXI., corresponding to the Experiments 
I, 2 and 3, represent the increase of the hydrobromic acid produced in the times Tj, 
&c. dra^vn on the abscissae. It is seen that these curves have also a point of contrary 
flexure, so that in the idio-chemical as well as the photo-chemical induction, a phase in 
the decomposition is found in which a maximum in the increase of the action occurs. 
The cause of this maximum increase appears therefore not to lie in any peculiarity which 
the light may possess, but rather in the mode of action of afiinity itself. 
The laws of photo-chemical induction which we have developed in this Part, explain 
most completely many of the singular phenomena which lie at the foundation of the 
photographic processes. Without entering into detail concerning the relations which 
these laws bear to the general processes of the photographer, we -^111 now merely con- 
sider one phenomenon which has been so difficult to explain, that it has been foimd 
necessary to assume the existence of certain rays endowed -with very peculiar properties, 
to which the name of “ rayons continuateurs ” has been given. This phenomenon was 
first established by Edmond Becqueeel from the following observations. If one-half 
of an iodized plate of silver, or a sensitive photographic paper, be protected from the 
action of light, whilst the other half is exposed to a constant source of light for such a 
time that no alteration, either perceptible to the eye or capable of being developed by 
photographic preparations, is efiected on the sensitive surface, the plate possesses the pro- 
perty, when exposed to a uniform but very slight amount of light, to blacken on the 
insolated half, whilst the part not pre\iously exposed remains unaffected. If, in the first 
short insolation, every part of the plate was not exposed to the same intensity of light, 
the blackening effected by the following uniform exposure corresponds to this intensity, 
so accm'ately indeed, that a daguerreotj^e picture commenced by this slight exposure 
may be developed by subsequent uniform insolation, almost as perfectly as if each 
amount of light represented on the picture, and not a constant amount, had acted for the 
whole time upon the plate. The explanation of this fact does not requii'e the assump- 
tion of a new class of rays, which cannot commence, but only continue, the photo-chemical 
