70 Intelligence and Miscellaneous Articles. 
In this, the glass compensating prism, instead of Fie. 2 
being mounted separately, is cemented upon one of sant 
the terminal faces of the compound spar-prism ; the gees 
angle of this latter, and also of the other terminal 
face, being suitably modified. 
This seems distinctly preferable to the original 
arrangement, for several reasons. 
1. The total length of the compound prism is 
rather less, being scarcely more than twice its breadth. 
2. The field is rather larger, so that the prism can 
be used over microscope eye-pieces (A andB) without 
any of the field of view being cut off. 
3. The whole arrangement is more compact, all the components 
being firmly cemented together, and therefore not liable to acci- 
dental displacement. 
4, There is less loss of light by reflection, the reflecting surfaces 
being reduced to two. 
A ray of light entering the prism in a direction parallel to its 
axis is divided into two rays; one of which, on emergence, follows 
a course parallel to that of the original incident ray, and is practi- 
cally free from distortion and colour; the other ray is deviated to 
the extent of about 59° 30’ (for yellow sodium-light), being, of 
course, strongly coloured and distorted. The angular separation 
is so great that this latter ray does not interfere with ordinary 
observations. 
L hope that the prism, which has cost me much time and labour, 
will meet with the approval of the Society, and take a place as a 
useful accessory to the microscope and other optical instruments.— 
Journal of the Royal Microscopical Society, August 1884. 
ON THE PENETRATION OF DAYLIGHT IN THE WATER OF THE LAKE 
OF GENEVA. BY MM. FOL AND ED. SARASIN. 
Questions relating to the absorption of light by more or less 
thick layers of the very pure water of the Lake of Geneva have 
been the object of a series of experiments, undertaken by a Com- 
mission of the Socicté de Physique et d’Histoire naturelle of 
Geneva, on the incentive, and under the direction, of M. Louis 
Soret*. 
We have been charged more particularly to examine, by means 
of photography, the extreme depth which daylight reaches. Our 
experiments consisted in exposing a photographic plate at various 
depths in that part of the lake where the water is deepest. 
We used Monckhoven’s rapid gelatino-bromide plates. They 
were placed in a special apparatus designed by one of us for these 
experiments. It consists of a brass photographie back, the two 
* Comptes Rendus, March 10, 1884, : 624, and Archives des Sciences 
physiques et naturelles, vol. xi. p. 827, vol. xii. p. 158, 1884. 
+ This apparatus was constructed according to the designs and instruc- 
tions of M. Fol by the Société génevoise d’instruments de Physique. 
oS ee eee 
