[Illustration: Fig. 9. Luminous Trees projected against the sky at sunrise.]
"Neither the hour of the day nor the angle which the object makes with the observer appears to have any effect; for on some occasions I have seen the phenomenon take place at a very early hour in the morning. Yesterday it was 10 A.M., when I saw it as represented in Fig. 10. I saw it again on the same day at 5 P.M., at a different place of the same mountain, for which the sun was just setting. At one time the angle of elevation of the lighted white shrubs above the horizon of the spectator was about 20°, while at another place it was only 15°. But the extent of the field of illumination is variable, according to the distance at which the spectator is placed from it. When the object behind which the sun is just going to rise, or has just been setting, is very near, no such effect takes place. In the case represented in Fig. 9 the distance was about 194 mètres, or 636 English feet, from the spectator in a direct line, the height above his level being 60 mètres, or 197 English feet, and the horizontal line drawn from him to the horizontal projection of these points on the plane of his horizon being 160 mètres, or 525 English feet, as will be seen in the following diagram, Fig. 10.
[Sidenote: SILVER TREES AT SUNRISE. 1858.]
[Illustration: Fig. 10. Luminous Trees projected against the sky at sunrise.]
[Sidenote: BIRDS AS SPARKS OR STARS. 1858.]
"In this case only small shrubs and the lower half of the stem of a tree are illuminated white, and the horizontal extent of this effect is also comparatively small; while at other places when I was near the edge behind which the sun was going to rise no such effect took place. But on the contrary, when I have witnessed the phenomenon at a greater distance and at a greater height, as I have seen it other times on the same and on other mountains of the Alps, large tracts of forests and immense spruce-firs were illuminated white throughout their whole length, as I have attempted to represent in Fig. 11, and the corresponding diagram, Fig. 12. Nothing can be finer than these silver-looking spruce-forests. At the same time, though at a distance of more than a thousand mètres, a vast number of large swallows or swifts (_Cypselus alpinus_), which inhabit these high rocks, were seen as small brilliant stars or sparks moving rapidly in the air. From these facts it appears to me obvious that the extent of the illuminated spots varies in a direct ratio of their distance; but at the same time that there must be a constant angular space, corresponding probably to the zone, a few minutes of a degree wide, around the sun's disk, which is a limit to the occurrence of the appearance. This would explain how the real extent which it occupies on the earth's surface varies with the relative distance of the spot from the eye of the observer, and accounts also for the phenomenon being never seen in the low country, where I have often looked for it in vain. Now that you are acquainted with the circumstances of the fact, I have no doubt you will easily observe it in some part or other of your Scotch hills; it may be some long heather or furze will play the part of our Alpine forests, and I would advise you to try and place a bee-hive in the required position, and it would perfectly represent our swallows, sparks, and stars."