TRANSLATIONS AND SELECTED ARTICLES. 47 
the horizontal light from the surface is totally reflected from the hy- 
pothenuse. Instead of this apparatus, Poggendorff proposed a glass 
mirror inclined at 45° to the horizon. I happened to have occasion 
to make a corresponding experiment ; I wanted to fill a tinplate tube, 
which was closed at the ends with glass plates and had a hole in the 
side, by placing’it in a very inclined position, quite under the sea 
level. When the upper glass plate had the right inclination, in the 
Tegernsee it reflected in sunny weather an emerald-green light more 
intense than I have obtained in any other way ; in the Achensee, how- 
ever, a blue light, as if it had passed through concentrated solution 
of sulphate of copper. Hence Arago’s proposal is; appropriate ; and 
if he had had an opportunity of carrying it out, he would certainly 
have given up the notion that water shows different colours in reflect- 
ed ani in transmitted light. 
The colour of water alters naturally, when solid particles are sus- 
pended init. By mixing such bodies which, like the above-mention- 
ed constituents of the ground, reflect red light mm preference when 
they are moistened, it may yet appear red; by greater masses of 
whitish sand which have become heaped up in the lakes during a con- 
tinuous storm, or which the rivers have worn down from their beds, 
the water appears clearer than otherwise. Simony observes that the 
Wolfang and Attersee appear in winter, when they are clearest, of a 
dark green, but in summer bluish-green or cerulean blue, and he con- 
siders this colour as occasioned more especially by the marl and grey 
sandstone predominant in the débris. 
In the previous considerations, the influence of the colour of fife 
sky and of the surrounding neighbourhood has been disregarded. 
Yet there are many who seek the cause of the colour of water in 
these circumstances. But these secondary influences must be taken 
into account along with the chief cause. When the surface of the 
lake is quite clear, it acts asa mirror. The special phenomena of 
colour are the more concealed, the more regular reflected light reaches 
the eye from the place in question; they appear purest where no or 
but little light is regularly reflected, for example, against a dark rocky 
background. But if the sea is in motion, the regular reflection 
always diminishes, and the aspect of the surface is changed by the 
occurrence of waves in a very complicated manner, depending on the 
formation of the bank, the direction and intensity of the wind, and 
