PHYSICAL SCIENCE. 



XXXV 



rentor appears to have been a Florentine, called 

 Salvino deyV Armato. He was of a Patrician 

 family. Maria Manni quotes the following in- 

 scription on his tomb, from an antiquarian 

 author : Qui diace (giace) Salvino d 1 Armato 

 degV Armati, Firenze invento de egl* Occhiali, 

 anno MCCCXVII. 



Two centuries later Maurolicus appeared. He 

 was acquainted with the crystalline lens of the 

 eye, and formed a correct judgment of the de- 

 fects of short-sighted and long-sighted eyes. 

 Baptisto Porta, a Neapolitan, invented the camera 

 obscura about the year 1560. The light was 

 admitted through a small hole in the window 

 shutter of a dark room, and gave an inverted 

 picture of the objects from which it proceeded 

 on the opposite wall. A lens was not employed 

 in the first construction of this apparatus, but 

 afterwards used ; and Porta went so far as to 

 consider how the effect might be produced with- 

 out inversion. 



Kepler was the first person who explained the 

 mechanical action of the eye in vision. He per- 

 ceived the exact resemblance of this organ to 

 the camera obscura, the rays entering the pupil 

 being collected by the crystalline lens and the 

 other humours of the eye into foci which paint 

 on the retina the inverted images of external ob- 

 jects. The mind perceives these images and re- 

 fers them at the same time to things without. 



Antonio a 1 Dominis, Archbishop of Spalatro, 

 was the first person who explained the pro- 

 duction of the rainbow. Having placed a bottle 

 of water opposite to the sun, and a little above 

 his eye, lie saw a beam of light issue from the un- 

 derside of the bottle, which acquired different 

 colours in the same order and with the same 

 brilliancy as the rainbow, when the bottle was a 

 little raised or depressed. From comparing all 

 the circumstances, he perceived that the rays had 

 entered the bottle, and that after two refractions 

 from the convex part, and a reflection from the 

 concave, they returned to the eye tinged with 

 different colours, according to the angle at which 

 the ray had entered. The book containing this 

 discovery was published in 1611. 



The telescope was invented about this time, 

 but somewhat earlier. The honour of the dis- 

 covery belongs to Middleburg, in Zealand. Two 

 different workmen, Zachariah Jans and John 

 Lapprey have each testimonies in their favour, 

 between which it is difficult to decide. The 

 former goes back to 1590, the latter comes down 

 to 1610. Zachariah Jans was an optician in 

 Middleburg, and Boreel has published a letter 

 from his son assigning 1590 as the date of the 

 invention, and another from his sister assigning 

 1610 as the date. From the account of Boreel 

 it would appear that Jans was the real inventor, 



and that the discovery of the telescope had been 

 preceded by that of the microscope. News of 

 this discovery was communicated to Galileo in 

 1610. He immediately constructed a telescope, 

 turned it to the heavens, and made the interest- 

 ing discoveries which have been already par- 

 ticularized. 



The theory of the telescope required that the 

 law of refraction should be discovered. This 

 discovery was the work of Snellius, a mathema- 

 tician of the Low Countries. To express this law 

 he supposed a perpendicular to the refracting 

 surface, at the point where the refraction is made, 

 and also another line parallel to this perpendicu- 

 lar at any given distance from it The refracted 

 ray as it proceeds, will meet this parallel, and 

 the incident ray is supposed to be produced till it 

 do so likewise. Now, the general truth which 

 Snellius found to hold, whatever was the position 

 of the incident ray, is, that the segments of the 

 refracted ray and of the incident ray, intercepted 

 by these parallels, had always the same ratio to 

 each other. 



In the triangle formed by the two segments 

 of the rays, and by the parallel which they inter- 

 sect, the said segments have the same ratio with 

 the sines of the opposite angles ; that is with the 

 sines of the angles of incidence and refraction. 

 The law, therefore, comes to this, that in the re- 

 fraction of light by the same mediums, the sine 

 of the angle of incidence has to the sine of the 

 angle of refraction always the same ratio. It 

 was first stated in this way by Descartes in his 

 Dioptrics, in 1637. But Snellius's law, which 

 comes to the same thing, had been publicly 

 taught in his lectures by professor -Hortensius, 

 and must therefore have been known to Des- 

 cartes. 



Descartes next entered upon an inquiry, in 

 which he was successful. In ordinary cases of 

 refraction, by spherical and other surfaces, the 

 rays are not collected into one point, but have 

 their foci spread over a certain surface, the sec- 

 tions of which are the curves called caustic 

 curves. The focus of opticians is only a point in 

 this surface, where the rays are more condensed, 

 and of course the illumination more intense than 

 in other parts of it. But if refraction is to be 

 employed either to produce heat or light, it would 

 be a great advantage to have all the rays which 

 come from the same point of an object united 

 accurately after refraction, in the same point of 

 the image. This led Descartes to inquire into 

 the figure which the superficies separating two 

 transparent media of different refracting powers 

 must have, that all the rays diverging from a 

 given point, might by refraction at the said 

 superficies be made to converge to another given 

 point. He showed that curves proper for gener- 



