GLASS-MAKING. 6 1 3 



reagents, make it one of the most useful of servants in the good 

 cause of science. 



First, then, a word in regard to its refractive power. 



If a beam of light pass from one medium to another of differ- 

 ent density, such as from air to water, its course is not altered, 

 provided the surfaces of the two media be at right angles to the 

 beam. A penny placed in a basin of water looks in no way dis- 

 torted if the eye be directly above the coin. But when a beam of 

 light passes into a second medium at other than a right angle its 

 course is bent. A straight stick, partly immersed in water, looks 

 crooked because the light reflected from the portion beneath the 

 water is bent on entering the air. The fact is familiar to every 

 one. This bending of the light has received the name of refrac- 

 tion, and its laws are exceedingly simple. If the beam pass into 

 a denser medium, as from air into water, the bending is toward 

 the perpendicular to the common surface of the two media. On 

 the other hand, if the passage be into a rarer medium, as from 

 water into air, the beam is bent away from the common per- 

 pendicular. We may, then, predict in a general way the course 

 of a beam of light when it changes its medium, but in scien- 

 tific work we must do better than that we must know the exact 

 course of the beam. This brings us to the second law of refrac- 

 tion, which is quite as simple as the first, but which requires, 

 if one is not mathematically inclined, a trifle more patience for 

 its comprehension. In any angle, if a perpendicular be dropped 

 from any point on one side to the opposite side, the ratio between 

 the perpendicular and the distance of its starting-point from the 

 apex of the angle will evidently be a constant quantity for that 

 angle, wherever the point be taken. This ratio is called the 

 sine of the angle. If one will take the trouble to draw a series 

 of angles from zero to ninety degrees, he will readily see that 

 the value of the sine increases from zero to unity, and that these 

 are its limits. 



Now, it is found by experiment that the ratio between the sine 

 of the angle of incidence (the angle which the impinging ray 

 makes with the common perpendicular to the two media) and the 

 sine of the angle of refraction (the angle which the refracted ray 

 makes with the common perpendicular) is a constant quantity. 

 This quantity is known as the index of refraction. 



But it may be asked what all this has to do with glass-making. 

 Essayists are prone to talk about evolution and the fourth dimen- 

 sion of space, and many other things which seemingly have no 

 connection with the subject in hand. In this case, however, the 

 wandering is justifiable, for the index of refraction is a constant 

 which must ever be borne in mind by the scientific glass-worker, 

 if he wishes to use the material in the construction of optical in- 



VOL. XXXIX. 



