12 Intelligence and Miscellaneous Articles. 
second day was strongly blackened, while plate 2 of the first day was no 
more affected than plate 4 of the second day. Finally, plate 7, exposed 
at 90 metres, was so acted upon that the lines which had been traced on 
the back were only imperfectly seen on the dark ground of the developed 
layer. 
Comparing the results obtained on the two days of the experi- 
ments, we are struck by the fact that the photographic action 
was greater on the 23rd September than on the 16th August. 
Weare thus led to conclude from these first attempts ; 
1. That daylight penetrates into the water of the Lake of Geneva 
to a depth of 170 metres, and probably further; that at this depth 
the illumination in full daylight is just comparable to that which 
we observe on a fine night when there is no moon. 
2. That at 120 metres the light is still very strong. 
3. That in September, in cloudy weather the light penetrates in 
larger quantity and to a greater depth than in August, in perfectly 
bright weather. Further experiments will have to decide whether 
this difference is to be attributed to the greater transparency of 
water in autumn and in winter, which the experiments of M. Forel* 
have put beyond doubt, or whether the light diffused from the 
clouds penetrates further than the more or less oblique rays of 
the sun. 
Previous to our experiments, M. Aspert had exposed gelatino- 
bromide plates in the Lake of Zurich at depths varying from 40 to 
90 metres. He immersed them at night, and withdrew them the 
following night. But the darkest night is still bright for a rapid 
gelatino-bromide plate. Our experiments seem then to be made 
under more trying circumstances. We intend to pursue: these 
experiments in the summer of 1885. 
We are desirous also, if possible, of making analogous experi- 
ments in the sea, where the greater transparency of the water leads 
to the supposition that the extreme limit of the luminous rays is at 
a still lower level. 
In this respect satisfactory data are wanting, for the experiments 
of the cruise of the ‘Porcupine’ have remained in the state of 
project, as the apparatus devised by Sir W. Siemens refused to act. 
The depth to which daylight penetrates in the sea is therefore still 
to be found.—Comptes Rendus, Noy. 10, 1884. 
* M. Forel investigated from 1873 the transparency of the waters of the 
Lake of Geneva by a photographic method ; he employed, however, albu- 
minized silver paper, which is less sensitive than gelatino-bromide. He 
immersed the sheets at night at various depths, and took them out after one 
or several twenty-four hours. This method gave for the limit of absolute 
darkness approximately 45 metres in summer and 100 metres in winter. 
The greater abundance of aquatic powders during summer, which is 
the cause of the greater opacity, is due to the thermal stratification of 
water during the hot season, from which results the power of holding in 
suspension powders of different densities (Archives des Sciences physiques 
et naturelles, vol. lix. p. 187, 1877). 
+ Archives des Sciences physiques et naturelles, vol. vi. p. 318, 1881. 
