[6] Memoires de la Soc. d’Aroncil, vol. ii.
[7] By a medium, in optics, is meant the space in which a ray of light moves, whether pure space, air, water, glass, diamond, or any other transparent substance through which the rays of light can pass in straight lines.
[8] Edinburgh Philosophical Journal for October 1819, p. 411.
[9] This mode of finding the focus of a concave lens may be varied as follows:—let the lens be covered with paper, having two small circular holes; and on the paper for receiving the light, describe also two small circles, but with their centres at twice the distance from each other of the centres of the circles. Then move the paper to and from, till the middle of the sun’s light, coming through the holes, falls exactly on the middle of the circles; that distance of the paper from the lens will be the focal length required.
[10] Small glass mirrors for performing some of the experiments, and illustrating some of the principles above alluded to,—may be made of the flattest kind of common watch glasses, by foliating or covering with tin leaf and quicksilver the convex surfaces of such glasses. Their focal distances will generally be from one to two inches. Such mirrors afford a very large and beautiful view of the eye, when held within their focal distance of that organ. Such mirrors will also serve the purpose of reflecting light on the objects viewed by microscopes. Larger mirrors, of from four to eight inches diameter, may be had of the optician at different prices varying from five to ten or fifteen shillings.
[11] Nicholson’s Journal of Natural Philosophy, &c. 4to. series, p. 225.
[12] There can be little doubt that some of the facts ascribed, in the western highlands of Scotland, to second sight, have been owing to the unusual refraction of the atmosphere; as one of the peculiarities attributed to those who possessed this faculty was, that they were enabled to descry boats and snips, before they appeared in the horizon.
[13] Fraunhofer was in the highest sense of the word, an Optician, an original discoverer in the most abstruse and delicate departments of this science—a competent mathematician, an admirable mechanist, and a man of a truly philosophical turn of mind. By his extraordinary talents, he was soon raised from the lowest station in a manufacturing establishment to the direction of the optical department of the business, in which he originally laboured as an ordinary workman. He then applied the whole power of his mind to the perfection of the achromatic telescope, the defects of which in reference to the optical properties of the materials used—he attempted to remedy; and by a series of admirable experiments, succeeded in giving to optical determinations, the precision of astronomical observations, surpassing, in this respect all who had gone before him, except perhaps, the illustrious Newton. It was in the course of these researches, that he was led to the important discovery of the dark lines which occur in the solar spectrum. His achromatic telescopes are scattered over Europe, and are the largest and best that have hitherto been constructed. He died at Munich, at a premature age, in 1826; his death, it is said being accelerated by the unwholesome nature of the processes employed in his glass-house; leaving behind him a reputation rarely attained by one so young. His Memoir “On the refractive and dispersive power of different species of glass, in reference to the improvement of Achromatic telescopes, and an account of the lines on the spectrum,” will be found in the “Edinburgh Philosophical Journal,” Vol. ix. pp. 288-299, and Vol. x. pp. 26-40, for 1823-4.
[14] Philosophical Transactions. Vol. 50. p. 294.
[15] Ecclesiasticus xliii. 11, 12.