Feb. 7, 1889] 



NATURE 



343 



PENETRATION OF DAYLIGHT INTO THE 

 WATERS OF THE GENEVAN LAKE AND 



INTO THE MEDITERRANEAN. 

 A SPECIAL COMMISSION appointed by the Society 

 -^*- of Physics and Natural History to study the colour 

 and transparency of the waters of the Lake of Geneva 

 have investigated the extreme limit reached by daylight 

 in the depths of the lake. 



Many naturalists have investigated this interesting 

 subject AiTiongst others, Prof. F. A. Forel, of Morges, 

 spent much time in investigating the Lake of Geneva. 

 Using paper specially prepared for photography, M. 

 Forel arrived at very interesting conclusions as to the 

 greater transparency of the water in winter than in 

 summer. But the paper was not sufficiently sensitive to 

 permit of the determination of the exact limit to which 

 the light of day extends in the depths of the lake. 



Another Swiss naturalist, M. Asper, engaged in the 

 same research, and examined several lakes, especially 

 Ziirich and Wallenstadt, for the extreme limit of the pene- 

 tration of daylight. He made use of dried photographic 

 plates of gelatine-bromide of silver. Proceeding in the 

 same way as M. Forel, he put his plates in in the night, 

 and took them out the following night, thus neglecting 

 the action exercised on the sensitive plate by the light 

 which still exists in the sky, even on a moonless night, 

 at the mo:nent of putting in and taking out. 



This action, however, should not be neglected, and its 

 neglect is the source of considerable error. The researches 

 of M. Asper were carried out in lakes where the waters are 

 less pure, and therefore less transparent, than those of 

 the Genevan lake ; they are therefore special to the lakes 

 which he studied, and do not bear upon our lake. M. 

 Asper, having found light at the lowest depth to which 

 he immersed his plates (140 metres), could not answer the 

 question as to the limit of penetration. 



1. On the Extreme Limit of the Penetration of Daylight 

 into the Waters of the Lake of Geneva. 

 The recent work, like that of M. Asper, consisted in 

 exposing photographic plates at various depths in the 

 deepest parts of the lake The rapid gelatine-bromide 

 plates of Munkhoven were used. A special apparatus, 

 warding off from the sensitive plate all light other than 

 that which really penetrated at the depth to which it was 

 plunged under the water, was added. This apparatus, 

 constructed by the Genevan Society for the Construc- 

 tion of Instruments in Physics, consists of a box of 

 rectangular shape, of brass, 40 centimetres in length and 

 20 in breadth, containing in the middle a sensitive plate 

 fixed by wedges. This frame is closed in the upper 

 part by two brass shutters, gliding into grooves with 

 double borders, separated from each other by means 

 of a strong spring contained in the bottom of the box, 

 so as to leave the plate entirely uncovered. At the 

 bottom of the frame a strong shaft is fixed in the form of 

 a J_, which bears the a.xes of rotation of two levers coupled 

 in the form of scissors. Each of the levers ends in 

 the upper part in a fork whose teeth pass from one side 

 of the frame to the other, and lean against two branches 

 which each of the shutters bears. Under the action 

 of the inner spring the shutters separate, and with them 

 the two arms of the lever. A weight is suspended at 

 the opposite extremities of the two levers, and acts on 

 them like the pressure of the fingers closing a pair of 

 scissors ; the two forks approach one another, and with 

 them the two shutters, which then cover the plate en- 

 tirely and defend it from any luminous action from with- 

 out. A hook fixed to one of the shutters, turns on the 

 apparatus when it is shut, and hangs in the groove of 

 the other shutter, preventing them thus from separating 

 during the transport from the dark chamber to the site 

 of the experiment ; it is raised only at the moment that 



the apparatus is attached to the sounding-line and 

 worked on by the weight ; as soon as the sounding-weight 

 reaches the bottom, it opens under the action of the 

 antagonistic spring ; it closes again immediately it is 

 withdrawn, and the weight leaving the bottom recom- 

 mences to act. The depth having been ascertained by 

 previous sounding, the length of the cord is regulated by 

 which the weight is suspended to the apparatus, 100, 

 200, 300 metres for instance, at the distance desired from 

 the surface of the water. After exposure for a certain 

 time, the apparatus is withdrawn and carried into the 

 dark room established on the ship in order to change the 

 plates, and, if necessary, to develop them immediately. 



The duration of the exposure has been about ten 

 minutes. The development has been effected by means 

 of oxalate of iron, with which the workers acted on each 

 plate for ten minutes. The plates were all covered by 

 the same emulsion. 



Experiments have been made near Evian, where the 

 lake presents a pretty wide plain at 310 metres depth. M. 

 Marcet was twice kind enough to put his steam yacht, the 

 Heron, at the disposal of the Commission ; and Prof. Forel, 

 of Morges, was kind enough, not only to lend his sounding- 

 line, but to aid by his advice and his experience. 



August 16, 1884, weather clear, sun bright: (<?) at 237 

 metres, two plates, one at 12.30, the other at 1.7 ; {b) at 

 1 13 metres, a plate at 2. 30 ; {c) at 300 metres a plate at 

 2.44. 



September 23, 1884, dull, but very clear weather, thin, 

 pretty luminous clouds, light wind varying from east to 

 north, we exposed : {d) at 147 metres, a plate at i o'clock 

 in the afternoon ; {e) at 170 metres, a plate at 2.26 ; (/) at 

 113 metres, a plate at 3.3 ; {g) at 90-50 metres, a plate 

 'It 3-34- 



For the sake of comparison, M. Fol had, on August 15, 

 at 10 p.m., exposed on a clear but moonless night : (//) a 

 plate in the open air for ten minutes ; {i) a plate in the open 

 air for five minutes. 



It was found that plate c (300 metres in depth) had re- 

 ceived no luminous impression whatever. It was the same 

 with plate <? (237 metres). Plate e, at 170 metres, was 

 lightly veiled, almost like plate /, exposed for five minutes 

 during the night. Plate d, at 147 metres, had been 

 vividly impressed, more than plate //, exposed in the night 

 for ten minutes. Of the two plates at 113 metres, the 

 plate / of the second day is much darkened, whilst plate 

 d of the first day is no more impressed than plate d of the 

 second day. Plate g, exposed at 90 metres, is so im- 

 pressed that characters which had been traced on the back 

 are only incompletely reserved on the dark background of 

 the developed layer. 



On comparing the results obtained in the two days of 

 experiment one is struck by the fact that the photographic 

 effect was much stronger on September 28 than on 

 August 16. 



From these two attempts the conclusions are drawn : — 

 (i) That daylight penetrates into the waters of the 

 Lake of Geneva in September at 170 metres depth, and 

 probably a little beyond that ; that at this depth the amount 

 of light in the day is almost simiUr to that perceived in a 

 clear moonless night. 



(2) That at 120 metres the action of transmitted light is 

 still very strong. 



(3) That in September, in dull weather, light penetrates 

 in greater abundance and more deeply into the water than 

 in .August in fine weather. 



Later experiments will show us whether this differ- 

 ence is to be attributed to the greater transparency of 

 water in autumn and winter, which the experiments of 

 M. Forel ascertained beyond doubt, or if the light diffused 

 by the clouds penetrates' more deeply than the more or 

 less oblique rays of the sun. 



Before these'cxperiments M. Asper had exposed plates 

 of gelatine-bromide in the Lake of Zurich at depths be- 



