Fig. 25.—Burning Mirror.
On the 3rd of April, at about two o’clock in the afternoon, the great mirror was mounted on its stand, and was found to be capable of setting a plank of wood on fire at a distance of 138 feet, when 128 glasses were used, although the light was weak at the time, and the sun was covered with mist. In pursuing these experiments great care had to be taken to prevent the by-standers placing themselves within range of its terrible power, for several were nearly blinded by looking at the brilliant focal point of the instrument. The next day, at eleven in the forenoon, although the sun was still covered with mist and fleecy clouds they were able to produce such a heat at 150 feet distant, with 154 glasses, that a pitched plank began to smoulder and would have burnt into flame had not the sun disappeared at that particular moment. On the fifth of April, at three in the afternoon, with the light much in the same weak condition as it was on the other days, they succeeded in igniting at 150 feet distant, a heap of shavings of deal mixed with charcoal and sulphur, in less than a minute and a half, with 154 glasses. When, however, the sun shone with its natural power, a few seconds were sufficient to effect these results.
On the 10th, when the sun was shining pretty powerfully, a pitched pine plank was easily fired with 128 glasses, at 150 feet distant. In this case the ignition was very sudden, and extended over the whole of the radiant spot forming the focus, which at the distance named measured 16 inches in diameter. The same day at half-past two, a pitched elm plank covered in some places with chopped wood, was set fire to with extreme rapidity, and burnt with such violence that it had to be dipped in water before it could be put out. In this experiment 148 glasses were used, at a distance of 150 feet.
On the 11th of April, the burning point was fixed at 20 feet distant from the mirror, and combustible substances were easily burnt with only 12 glasses. With 21 glasses a half-burnt elm plank was set fire to, and with 45 a piece of tin weighing six pounds was almost immediately melted. Silver sheet was fused, and an iron plate was made red-hot with 117 glasses. In giving an account of these interesting experiments, Buffon expresses his conviction that at 50 feet it would have been easy to have melted metals if all the glasses of the mirrors had been used. When used at that distance, the burning spot was six to seven inches in diameter. He also noticed that when metals were melted, part of them were dissipated in brilliant vapour, which was so thick as to cast a shadow on the ground, although it seemed to be as bright as the sun itself. When the sun was at its full strength, and all the glasses were brought into requisition, wood was set on fire at a distance of over 200 feet, and metals and minerals were fused at 40 and 50 feet. Hence the possibility of making and using these mirrors as Archimedes was said to have done, was proved practically by the great naturalist. [Fig. 25] represents a burning mirror in action.
Robertson, an English philosopher, residing in France during the days of the first Republic, reconstructed the mirrors described by historians as being used by Archimedes, and the results he obtained were thought sufficiently important by the Council of the Department of Ourthe to merit an attentive examination by two members of their body, who reported in favour of their being used as instruments of war.
It would be possible to pursue this subject still further, and give an account of numerous experiments made on burning mirrors by various philosophers, but we must not forget that it is light and heat that we have more especially to deal with in the present work. Already we have possibly strayed from our path a little too far, but the two influences are so closely connected with each other that it is almost impossible to speak of them separately when reflection is in question.
CHAPTER VII.
LENSES.
The word lens is derived from the Latin name of the seed of the Ervum lens, or ordinary lentil. When eating this wholesome vegetable, almost every one has noticed that its shape is exactly that of a double convex lens, as represented in the following figure:—