OPTICS. 357 



Concerning the nature, or essence, of light, two different theories 

 have long been maintained. The emissive theory, supposes light to 

 consist of material, though imperceptibly small particles, or atoms, 

 thrown off from the luminous body, and diverging in all directions. 

 This theory was maintained, in ancient times, by Pythagoras ; and 

 was adopted by Newton. The undulatory theory, supposes light 

 to be caused by a peculiar fluid, or ethereal medium, diffused through- 

 out all nature ; in which vibrations are produced by luminous bodies, 

 like those in the air by sounding bodies ; only, far more rapid, and 

 sensible only to the eye. This theory was proposed by Huyghens ; 

 and was advocated by Euler, and Young. Either of these theories 

 may serve to explain most of the facts, and assist in remembering 

 them : but the preference is now more generally given to the latter. 

 The study of Optics, has served not only to aid the sight, by the in- 

 vention of various instruments ; but also to explain many phenomena 

 of nature, which were previously unknown, or involved in mystery, 

 or applied to purposes of deception and crime. 



The history of Optics, commences, perhaps, with the mention of 

 brazen looking glasses, in the books of Exodus and Job, as in use 

 among the ancient Hebrews ; and of burning lenses, of glass or 

 crystal, as known in Greece, about 450 B. C. Archimedes is said to 

 have set fire to the Roman fleet attacking Syracuse, by means of an 

 assemblage of glass mirrors. The earliest systematic writer on 

 Optics, was Euclid, the geometer ; who adopted the notion of Empe- 

 docles and Plato, that light proceeds originally from the eye, and is 

 then reflected back from luminous objects. The prismatic spectrum, 

 was known to Seneca; but his explanation of it was imperfect. 

 Ptolemy first gave a table of refractions, in his work on Optics, and 

 applied it to the correction of astronomical observations. Alhazen, 

 the Arabian, who wrote about A. D. 1100, disproved the Platonic 

 notion of ocular beams ; and adopted the emissive theory. 



Roger Bacon, the English monk, first discovered the principle of 

 microscopes and telescopes, and probably invented spectacles ; which 

 were first used about 1275, during his life time. The invention of 

 the camera obscura, by Porta of Naples, about 1460, led Kepler to 

 discover the true mechanism of the eye. The invention of the 

 telescope, is attributed, by Descartes, to Metius of Holland, about 

 1600; and by others, to Jansen, or Johnson, of Zealand, about the 

 same date : but the English claim the invention for Leonard Digges, 

 as early as 1591. Galileo, having heard of this instrument, invented, 

 in 1609, the telescope which bears his name. The astronomical 

 telescope, was suggested, or revived, by Kepler, and made by Schei- 

 ner, about the year 1650 : the reflecting telescope was first con- 

 structed by James Gregory, in 1663; the Newtonian, was invented 

 in 1666; and the Cassegrainian, in 1672. The invention of the 

 simple microscope, has been attributed to Drebbell, of Holland, about 

 1618 ; but we think more justly to Jansen, about 1610. The com- 

 pound refracting microscope, was invented by Fontana of Naples, in 

 1618. The magic lantern was invented by Kircher, who died in 

 1680. 



In 1611, Antonio de Dominis, archbishop of Spalatro first illus- 



