SIX LECTURES ON LIGHT. 



glass capable of rotation. A beam of light 

 is received upon the glass and reflected back 

 upon the line of its incidence. Though the 

 incident and the reflected beams pass in 

 opposite directions, they do not jostle or dis- 

 place each other. The index being turned, 

 the mirror turns along with it, and at each 

 side of the index the incident and the 

 reflected beams are seen tracking themselves 

 through the dust of the room. The mere 

 inspection of the two angles enclosed be- 

 tween the index and the two beams suffices 

 to show their equality. The same simple 

 apparatus enables us to illustrate a law of 

 great practical importance, name y, that, 

 when a mirror rotates, the angular velocity 

 of a beam reflected from it is twic^ that of 

 the reflecting mirror. One experiment will 

 make this pla n to you. The mirror is 

 now vertical, and both the incident and the 

 reflected beams are horizontal. Turning the 

 mirror through an angle of 45 the reflected 

 beam is vertical ; that is to say, it has moved 

 9O r , or through twice the angle of the mirror. 



One of the problems of science, on which 

 scientific progress mainly depends, is to help 

 the senses of man by carrying them into re- 

 gions which could never be attained without 

 such help. Thus we arm the eye with the 

 telescope when we want to sound the depths 

 of space, and with the miscroscope when we 

 want to explore motion and structure in their 

 infinitesimal dimensions. Now, this law of 

 angular reflection, coupled with the fact that 

 a beam of light possesses no weight, gives us 

 the means of magnifying small motions to an 

 extraordinary degree. Thus, by attaching 

 mirrors to his suspended magnets, and by 

 wa ching the images of scales reflected from 

 the mirrors, the celebrated Gauss was able to 

 detect the slightest thrill or variation on the 

 part of the earth's magnetic force. The mi- 

 nute elongation of a bar of metal by the mere 

 warmth of the hand may be so magnified by 

 this method as to cause the index-beam to 

 move from the ceiling to the floor of this 

 room. The elongation of a bar of iron when 

 it is magnetized may be thus demonstrated. 

 By a similar arrangement the feeble attrac- 

 tions and repulsions of the diamagnetic force 

 have been made manifest; while in Sir William 

 Thompson's reflecting galvanometer the prin- 

 ciple receives one of its latest applications. 



For more than 1,000 years no step was 

 taken in optics beyond this law of reflection. 

 The men of the Middle Ages, in fact, endeav- 

 ored on the o ,e hand to develop the laws of 

 the universe out of their own consciousness, 

 while many of them were so occupied with 

 the concerns of a future world that they 

 looked with a lofty scorn on all things pertain- 

 ing to this one. Notwithstanding its demon- 

 strated failure during 1,500 years of trial, 

 there are still men among us who think the 

 riddle of the universe is to be solved by this 

 appeal to consciousness. And, like most 

 people who support a delusion, they maintain 



theirs warmly, and show scant respect for 

 those who dissent from their views.* As re- 

 gards the refraction of light, the course of 

 real inquiry was resumed in noo by an Ara- 

 bian philosopher named Alhazen. Then it 

 was taken up in succession by Roger Bacon, 

 Vitellio, and Kepler. One of the most im- 

 portant occupations of science is the deter- 

 mination, by precise measurements, of the 

 I quantitative relations of phenomena. The 

 value of such measurements depends upon the 

 skill and conscientiousness of the man who 

 makes them. Vitellio appears to have been 

 both skilful and conscientious, while Kepler's 

 nabit was to rummage through the ob.-erva- 

 tions of his predecessors, look at them in ail 

 lights, and thus distill from them the princi- 

 ples which united them. He had done this 

 with the astronomical measurements of 

 Tycho Brahe, and had extracted from them 

 the celebrated " laws of Kepler." He did k 

 also with the measurements of Vitellio. ^3ut 

 in the case of refraction he was not success- 

 ful. The principle, though a simple one, es- 

 caped him. It was firs discovered by \Ville- 

 brod Snell, about the year 1621. 



Less with the view of dwelling 1 upon the 

 phenomenon itself than of introducing it to 

 you in a form which will render intelligible 

 the play of theoretic thought in Newton's 

 mind, I will show you the fact of refraction. 

 The dust of the air and the turbidity of a liquid 

 may here be turned to account. A shallow 

 circula- vessel with a glass face, half rilled 

 with water, rendered barely turbid by the 

 precipitation of a little mastic, is placed upon 

 its edge with its glass face vertical. Through 

 a slit in the hoop surrounding the vessel a 

 beam of light is admitted. It impinges upon 

 the water, enters it, and tracks itself through 

 the liquid in a sharp, bright band. Meanwhile 

 the beam passes unseen through the air above 

 the water, for the air is not competent to 

 scatter the light. A puff of tobacco smoke 

 into this space at once reveals the track of the 

 incident-beam. If the incidence be vertical, 

 the beam is unrefracted. If oblique, its re- 

 fraction at the common surface of. air and 

 water is rendered clearly visible. It is also 

 seen that reflection accompanies refraction, 

 the beam dividing itself at the point of inci- 

 dence into a refracted and a reflected portion. 

 The law by which Snell connected together 

 all the measurements executed up to his time, 

 is this : Let A B C D represent the outline 

 of our circular vessel (Fig. i), A C being the 

 water-line. When the beam is incident along 

 B E, which is perpendicular to A C, there is 

 no refraction. When it is incident along m 

 E, there is refraction : it is bent at E and 

 strikes the circle at n. When it is incident 



* Schelling thus expresses his contempt for experi- 

 mental knowledge : " Newton's Optics is the greatest 

 illustration of a whole structure of fallacies, which in 

 all its parts is founded on observation and experi- 

 ment." There are some small imitators of Schelling 

 still in Germany. 



