658 
Frocecdings of the Royal Society 
miich noticed, because it casts but little shadow, on account of the 
light coining from a wide area. To satisfy ourselves, however, of the 
brilliancy of the western sky, compared with the north, south, 
or east, we have only to project, by means of a mirror, a small 
area of the western sky on to the eastern. Compared with the 
reflection of the western sky, the eastern looks black, and the pro- 
jected image contrasts as strongly with the eastern sky as the sun 
does with the heavens at mid-day. 
There is another way of studying the illuminating power of the 
western sky, which I had occasional opportunities of following. 
On some evenings there were a few small clouds floating about, 
just enough of them to show the successive illuminations, but not 
so many as to cast shadows, and interfere with each other. These 
test reflectors were carefully watched as the sun went down and till 
it was nearly dark. As the sun touched the horizon, it shone into 
my room, and painted an image of the window on the ojrposite 
wall in bright orange light. At the same moment it lighted up the 
little clouds with the same coloured light. This colour deepened 
on the clouds as the sun sunk below the horizon, till they glowed 
with a fine red light. After a little the sun had sunk so low that 
it ceased to shine on the clouds, their brilliancy died away, and at 
last they looked black. The sky overhead, however, still remained 
brilliantly illuminated. After a time a change began to appear ; as 
the light in the sky died away the clouds lost their dark look, and 
gradually after a time appeared to be lighted up again; till at last 
their western edges again glowed ivith a red light, in gene- 
ral appearance very much the same as the first red illumination. 
This time obviously the source of the illumination was the western 
sky, as the clouds were much too low to catch the direct rays of 
the sun. The boundaries of the illuminated parts were no longer 
sharp, as when the sun shone on them, but hazy and indefinite on 
account of the light coming to them from the wide area of the 
western sky. 
So far as I have been able to judge, the second glow which illumin- 
ated the whole heavens was produced in exactly the same way as 
the second illumination of the clouds as above described. On some 
evenings thin hazy clouds floating in the sky, a little below the 
haze that gave the true glow, were brilliantly illuminated with red 
