344 



NATURE 



[Fed. 7, 1889 



tween 40 and 90 metres, in the Lake of Wallenstadt from 

 90 to 140, and he obtained an effect from all. He put 

 them in in the night, left them exposed a whole day, and 

 took them out the following night. But, as the ex- 

 posures of plates // and / have shown, the darkest night is 

 still light for a plate of rapid gelatine-bromide. 



2. On the Exf rente Limit of the Penetration of Daylight 



into the Waters of the Mediterranean Sea. 



After being assured by the experiments in the Genevan 

 lake that their apparatus worked well, the Commission 

 desired to make similar experiments in the sea, in which 

 the greater transparency of the water would lead one to 

 suppose that the extreme limit of the luminous rays would 

 be at a still lower level. 



No satisfactory experiments had yet been made in re- 

 gard to this, for the experiments of the Porcupine were 

 not carried out, M. Siemens's apparatus refusing to work. 



Owing to the kindly mediation of Dr. J. Barrois, 

 Director of the Zoological Station of Villefranche-sur-Mer, 

 the Albatros was put at the disposal of the Commission 

 for several days in the spring of 1885. 



The method of procedure was the same as for the 

 experiments in the lake, only that it was important to 

 preserve the sensitive plate against the chemical action 

 of the salt water by adding a thick layer of varnish to 

 the bitumen. The luminous impression was made by 

 the back of the plate and through the thickness of the 

 glass. Repeated washings with essence of turpentine and 

 alcohol sufficed to remove the varnish before proceeding 

 to the development. As before, oxalate of iron was used. 



The experiments took place on March 25 and 26, 1885, 

 and were favoured by calm and fine weather. The depths 

 wanted were found near Cape Ferrat, from 400 to 600 

 metres. 



A. From 10.30 to 10.40, plate exposed at the depth of 

 200 metres to start with. 



B. From 12.45 to 12.50, at a depth of 280 metres. 



C. From 1 1.30 to 1 1.40, at a depth of 345 to 350 metres. 



D. From 10.55 to 11. 5, at a depth of 360 metres. 



E. From 10.15 to 10.25, at a depth of 380 metres. This 

 experiment took place under exceptionally favourable 

 circumstances ; there was no breeze, the ship remained 

 perfectly stationary, and the line perfectly vertical. 



F. From 1.20 to 1.30, cloudy but pretty light, at a 

 depth of 405 to 420 metres. All these plates except F 

 were exposed during bright sunshine. 



Plates A and B were found to be very much impressed. 

 On the plates C, D, and E the strength of the impression 

 diminishes very regularly with the increase of depth. On 

 plate E the strength of the impression is notably inferior 

 to that of an exposure of the same duration in the air, on 

 a clear moonless night. It may be compared to that of a 

 shorter exposure, five minutes only, in the latter conditions. 

 Plate F does not bear the least trace of any impression 

 whatever. It is no doubt to be regretted that this last 

 experiment did not take place, like the others, in clear 

 weather. But the degree of the impression of plate 

 E, of 380 metres, is already so weak that it may be 

 pretty safely concluded that the extreme limit could 

 not be more than 20 metres lower. On the other hand, 

 the experiments in the Lake of Geneva have shown that 

 the dispersion of the sunlight by a light layer of clouds 

 does not bring about a notable diminution in the depth 

 which it may attain in the water. 



It is concluded, then, from these experiments, that in 

 the month of March, in the middle of the day, with a 

 bright sun, the last rays of daylight stop at 400 metres 

 from the surface in the Mediterranean. 



3. Effects of the Seasons on the Limit of Penetration of 



Daylight in the Waters of the Lake of Geneva. I 



The experiments of M. Forel, mentioned above, showed 

 that photographic paper dipped in the lake is blackened 



in winter to a depth of 100 metres, whilst in summer it is 

 not blackened beyond 45 metres. It is interesting to know 

 whether this variation of transparency with the season 

 belongs only to superficial layers, or if the same law holds 

 good also at lower levels. 



March 18, 1885, the Commission went into the middle 

 of the lake on the Sachem, steam yacht of M. E. Reverdin, 

 which its owner put at their disposal. As in former expe- 

 riments on the lake, M. Forel was present. The weather 

 was pretty clear ; a light layer of clouds dispersed the 

 light without arresting completely the direct rays of the 

 sun. The following plates were exposed: {k) from 9.20 to 

 9.30, at 158 metres ; (/) from lo.o to 10.10, at 192 metres ; 

 (;//) from 10.30 to 10.40, at 235 metres ; («) from 10.10 to 

 11.20, at 240 to 245 metres ; {o) from 11.48 to 12.23, at 

 280 to 300 metres. 



The duration of exposure was uniformly ten minutes for 

 all, save the last, which remained uncovered, at 280 metres, 

 for 35 minutes. In spite of that, not the least trace of im- 

 pression was visible either on this plate or on plates m 

 and n. The plate / was very faintly impressed, almost 

 like plate E, of 380 metres in the sea. Plate k, at 158 

 metres, is of nearly the same force as C. 



These experiments show that the extreme limit of the 

 action of daylight in the lake in winter is a little beyond 

 200 metres. 



A comparison between this series of experiments and 

 the preceding shows that the light only descends 20 or 

 30 metres lower in March than in September ; the differ- 

 ence is perhaps a little more considerable in the month 

 of August. The layers of water situated below 100 metres 

 escape the law of variation of transparency established by 

 M. Forel for the more superficial layers ; the variations of 

 temperature accompanying the seasons, on the effect ot 

 which M. Forel bases his theory, not being sensibly felt 

 beyond a certain depth. 



Compared to the series of plates exposed in the lake, 

 the series brought from the Mediterranean is striking by 

 its slower and more regular gradation. This gives rise 

 to the idea that whilst in the lake the light is promptly 

 intercepted by deep layers more or less troubled, in the 

 Mediterranean the absorption of the pure water would be 

 the principal if not the only factor in the arrest of the 

 luminous rays. 



4. On the Penetration of Light in the Depth of the Sea 

 at Differeftt Times in the Day. 



The preceding experiments led to very exact deter- 

 minations of the extreme limit of the penetration of day- 

 light in the waters of the Lake of Geneva and in those of 

 the Mediterranean. 



The following year, pursuing the same kind of experi- 

 ments, the relations which exist between the depth to 

 which light reaches in the water and the inclination of 

 the sun or the variations in the amount of light were 

 investigated. 



The Report continues : — 



" As we no longer sought a single limit, but a series of 

 limits at fixed times during the day, we required a series 

 of plates exposed at the same instant at different depths 

 and capable of comparison with one another. Instead of 

 a single large apparatus, like that which we had been 

 using, we used twelve small ones, constructed on the 

 same principle, which we placed at regular intervals of 

 20 metres along the cord. These apparatus were also 

 furnished by the Genevan Society for the Construction of 

 Instruments in Physics. They consist of a little rectan- 

 gular frame of brass, in which glides in double grooves 

 the drawer containing the sensitive plate, and which an 

 interior spring tends always to open. The frame is fixed 

 by two rings of brass, which at their upper part allow 

 the axis of rotation of a lever to pass. The apparatus 

 is suspended to the sounding-cord by an arm of this bent 

 lever, whilst the lower arm acts on a spring fixed to the 



