ON THE HISTORY OF OPTICS. 375 



says, is the action of a transparent substance ; and if there were absolutely 

 no medium between the eye and any visible object, it would be absolutely 

 impossible that we should see it. 



It is said that Archimedes made a compound burning mirror, of suffi- 

 cient power to set on fire the Roman ships ; in this form the story is 

 scarcely probable, although the possibility of burning an object at a great 

 distance by a collection of plane mirrors has been sufficiently shown by 

 the experiments of Buffon.* It is, however, not unlikely that Archimedes 

 was acquainted with the properties of reflecting surfaces,t and that he 

 confirmed his theories by some experimental investigations. The work on 

 catoptrics, attributed to Euclid, contains the determination of the effects of 

 reflecting surfaces of different forms ; but it is not supposed to be genuine. 

 The existence and the magnitude of the atmospheric refraction were well 

 known to Ptolemy, and a treatise of this astronomer on the subject is still 

 extant in manuscript. 



The mathematical theory of optics, or the science of dioptrics and 

 catoptrics, made some advances in the middle ages from the labours of 

 Alhazen and Vitellio.;j: Alhazen was mistaken in some of his proposi- 

 tions respecting refraction ; Vitellio, a native of Poland, gave a more 

 correct theory of this subject, and constructed a table of refractive densi- 

 ties, showing the supposed proportions of the angles of incidence and 

 refraction in the respective mediums. 



The invention of the magic lantern is attributed to Roger Bacon, and 

 the lens was soon afterwards commonly applied to the assistance of de- 

 fective sight. It has been much disputed whether or no Bacon was ac- 

 quainted with telescopes ; the prevalent opinion is, that the passages, 

 which have been alleged to prove it, are insufficient for the purpose ; but 

 there is reason to suspect, from the testimony of Recorde,|| who wrote in 

 1551, not only that Bacon had actually invented a telescope, but that 

 Recorde himself knew something of its construction. Digges also, in a 

 work published in 1571, IF has a passage of a similar nature, and from 

 Bacon's own words it has been conjectured that an instrument resem- 

 bling a telescope was even of much higher antiquity. But the first person 

 who is certainly known to have made a telescope, is Janson, a Dutchman, 

 whose son, by accident placing a concave and a convex spectacle glass at 

 a little distance from each other, observed the increased apparent mag_ 

 nitude of an object seen through them ; the father upon this fixed two 

 such glasses in a tube a few inches long, and sold the instrument in this 

 form.** He also made some telescopes of greater powers, and one of his 



* Ph. Tr. 1748, p. 504. 



f See Kircher's Ars Magna Lucis et Umbrae, 4to, 1646. Parsdhs, Ph. tr. 

 1754, p. 621. 



+ Opticse Thesaurus per Risnerum, fol. Basle, 1572. 



Kepler, in his Paralipomena ad Vitellionem, 4to, Frankf. 1604, laboured un- 

 successfully to discover the true law of refraction. 



|| See Ph. Mag. xviii. 245. 



^f Pantometria. 



** Borellus, De Vero Telescopii Tnventore, 4to, Hagse, 1655. 



