light of the sun shine through an angular piece of glass, it will give all the colours of the rainbow. But, without investigating the nature of the phenomenon, he contents himself with saying, that this appearance is not of any real, but only a species of false colour, such as is seen in the neck of a pigeon, which We thus find that the changes with the position. refraction, as well as the reflexion of light, had not escaped the notice of the ancients. In a treatise on Optics, ascribed to Euclid, there is an attempt made to explain the phenomenon of the image of an object appearing as if it were suspended in the air, between the spectator and a concave mirror; and also an attempt to determine the size and figure of objects, from the angle under which they appear, or that the extremities of them subtend at the eye. The magnifying power of concave mirrors is mentioned both by Seneca and Pliny. It is probable that the ancient Romans and Druids had a method of lighting their sacred fire by means of reflecting concave speculums, and it is related by historians that Archimedes burned the Roman fleet by mirrors. Ptolemy, who lived about 150 years after Christ, was acquainted with atmospheric refraction, and of its being the cause of the sun, moon, and stars appearing higher in the heavens than they would otherwise do. From the days of Ptolemy down to the time of Alhazen, an Arabian philosopher who lived in the twelfth century, a great chasm is found in the history of optics. Alhazen gives a tolerable description of the eye, and treats largely of the nature of vision; maintaining that the crystalline humour is of principal use for this purpose, but without considering it as a lens ; and asserting that vision is not completed till the impressions of external objects are conveyed by the optic nerve to the brain. He accounts for single vision by two eyes by supposing that when two corresponding parts of the retina are affected, the mind perceives but one image; and he first advanced the opinion that the stars are sometimes seen above the horizon by means of refraction, when they are really below it, and also that the cause of the twinkling of the stars is refraction. From the writings of Alhazen and some imperfect experiments of Roger Bacon subsequently made, it is probable that the construction of spectacles was hit upon by Salvinus Armatus, a nobleman of Florence, who died in 1317. In the year 1311 a work was written by Theodoric, in which a rational explanation of the double rainbow is given. In 1575 a treatise called "De Lumine et Umbrâ" was published by Maurolycus, teacher of mathematics at Messina, in which he demonstrates that the crystalline humour of the eye is a lens that collects the rays of light issuing from external objects, and throws them upon the retina. He showed that the defects called long-sightedness and short-sightedness proceeded from too small or too great a refracting power in the eye; and that in the former case the pencils of rays do not converge fast enough, so that the foci are beyond the retina; and in the latter that the rays converge too fast, and come to a focus before they reach the retina; and further showed how and why these defects were remedied by the use of convex and concave lenses. He failed to discover the formation of the picture of external objects on the retina, which discovery was afterwards first made by Kepler in 1604. About the time that Maurolycus made his discovery of the nature of vision, Baptista Porta, a Neapolitan philosopher, invented the camera-obscura, which threw still more light on the same subject. The invention of the camera-obscura suggested to Kircher the invention of the magic lantern, which does that in the night that the camera does in the day. Porta observed that the pupil is contracted involuntarily when it is exposed to a strong light, and expands when the light is small. He was mistaken, however, in his opinion concerning the cause of single vision with two eyes, for he states we never see with more than one eye at one time. The accumulated facts and experiments furnished by various scientific men, and the numerous suggestions of writers on optics, on the construction and use of lenses, and their combinations, had now prepared the way for the construction of telescopes and microscopes. The approach to the construction of the telescope was so gradual that the honour of its invention cannot be exclusively ascribed to any one person. It is, however, generally admitted that to Jansen, a spectaclemaker of Middleburgh, the greater share of the credit is due. The first telescope was made by him in 1590. He had no sooner found the arrangement of lenses that produced the desired effect than he enclosed them in a tube, and ran with his instrument to Prince Maurice of Nassau, who immediately conceiving that it might be of use to him in his wars, desired the maker to keep it a secret. But this was found impossible, though attempted for some time. Among those who applied the telescope to the great ends of astronomical science, the name of Galileo stands foremost. He made a telescope himself which magnified about thirty times, and with which he discovered the satellites of Jupiter, the solar spots, and that the milky way and nebula consisted of a vast number of fixed stars, which, on account of their great distance or extreme smallness, were invisible to the naked eye. Subsequently he discovered that the planet Venus changes her phases like the moon. Kepler suggested important improvements in the construction of telescopes; he also very clearly explained in a most scientific manner the principles of the instrument. He attributed erect vision from an inverted image on the retina to an operation of the mind beyond our power to understand. To him also is due the discovery of the great law of motion of the heavenly bodies, viz., that the squares of the periodical times are as the cubes of the mean distances from the bodies about which they revolve. At the period to which we now refer, the beginning of the seventeenth century, the subject of the refraction of the atmosphere received a great deal of attention from scientific men, particularly from Tycho Brahe, who, perceiving the importance of it to the perfection of astronomy, applied himself diligently to it. He determined the amount of atmospheric refractions, at certain altitudes, to a tolerable degree of correctness. The honour of the discovery of the law of refraction, like many other important discoveries, cannot be exclusively ascribed to any one person, undoubtedly Snellius deserves a large share of the honour; it is to Descartes that we are indebted for the best exposition of the law of refraction. CHAPTER II. REFRACTION OF LIGHT. ALTHOUGH a ray of light will always move in the same straight line when it is not obstructed, yet many persons must have noticed that when the light falls on a drop of water, or a piece of glass, or a vial containing any fluid which allows the light to pass, it does not reach the eye or illuminate a piece of paper placed behind those bodies in the same manner as before they were put in its way. This evidently is caused by some power which resides in the body of changing the direction of the light. The branch of optics that explains the law according to which the direction of the light is thus changed is called dioptics, from two Greek words, one of which signifies through, and the other to see, because the bodies which cause this change in the direction of light are those through which we can see or through which light passes. In order to illustrate how this change or bending of light is produced, let w x y z (Fig. 1) represent a W Y E vessel, in one side of which, x z, there is a small hole h. |