472 



NATURE 



[September 17, 1896 



Physics can be taught so as to be a suVjject of the greatest 

 possible educational value, but when it is so it is not so much 

 because the student acquires a knowledge of a number of 

 interesting and important facts, as by the mental training the 

 study affords in, as Maxwell said, "bringing out theoretical 

 knowledge to bear on the objects and the objects on our 

 theoretical knowledge." I think this training can be got better 

 by going very slowly through such a subject as mechanics, 

 making the students try innumerable experiments of the simplest 

 and, what is a matter of importance in school teaching, of the 

 most inexpensive kind, but always endeavouring to arrive at 

 numerical results, rather than by attempting to cover the whole 

 range of mechanics, light, heat, sound, electricity, and magnet- 

 ism. I confess I regret the presence in examinations intended 

 for school boys of many of these subjects. 



I think, too, that in the teaching of physics at our universities, 

 there is perhaps a tendency to make the course too complex and 

 too complete. I refer especially to the training of those students 

 who intend to become physicists. I think that after a student 

 has been trained to take accurate observations, to be alive to 

 those pitfalls and errors to which all experiments are liable, 

 mischief may in some cases be done if, with the view of learning 

 a knowledge of methods, he is kept performing elaborate 

 experiments, the results of which are well known. It is not 

 given to many to wear a load of learning lightly as a flower. 

 With many students a load of learning, especially if it takes a 

 long time to acquire, is apt to crush enthusiasm. Now, there is, 

 I think, hardly any quality more essential to success in physical 

 investigations than enthusiasm. Any investigation in experi- 

 mental physics requires a large expenditure of both time and 

 patience; the apparatus seldom, if ever, begins by behaving as 

 it ought ; there are times when all the forces of nature, all the 

 properties of matter, seem to be fighting against us ; the instru- 

 ments behave in the most capricious way, and we appreciate 

 Coutts Trotter's saying, that the doctrine of the constancy of 

 nature could never have been discovered in a laboratory. These 

 difficulties have to be overcome, but it may take weeks or 

 months to do so, and, unless the student is enthusiastic, he is 

 apt to retire disheartened from the contest. I think, therefore, 

 that the preservation of youthful enthusiasm is one of the most 

 important points for consideration in the training of physicists. 

 In my opinion this can best be done by allowing the student, 

 even before he is supposed to be acquainted with the 

 whole of physics, to begin some original research of a 

 simple kind under the guidance of a teacher who will en- 

 courage him and assist in the removal of difficulties. If the 

 student once tastes the delights of the successful completion of 

 an investigation, he is not likely to go back, and will be better 

 equipped for investigating the secrets of nature than if, like the 

 White Knight of "Alice in Wonderland," he commences his 

 career knowing how to measure or weigh every physical quantity 

 under the sun, but with little desire or enthusiasm to have any- 

 thing to do with any of them. Even for those students who 

 intend to devote themselves to other pursuits than physical in- 

 vestigation, the benefits derived from original investigation as 

 a means of general education can hardly be over-estimated, the 

 necessity it entails of independent thought, perseverance in 

 overcoming difficulties, and the weighing of evidence gives it an 

 educational value which can hardly be rivalled. We have to 

 congratulate ourselves that, through the munificence of Mr. 

 Ludwig Mond, in providing and endowing a laboratory for 

 research, the opportunities for pursuing original investigations 

 in this country have been greatly increased. 



The discovery at the end of last year by Prof. Rcintgen of a 

 new kind of radiation from a highly exhausted tube through 

 which an electric discharge is passing, has aroused an amount 

 of interest unprecedented in the history of physical science. 

 The effects produced inside such a tube by the kathode rays, the 

 bright phosphorescence of the glass, the shadows thrown by 

 opaque objects, he deflection of the rays by a magnet, have, 

 thanks to the researches of Crookes and Goldstein, long been 

 familiar to us, but it is only recently that the remarkable effects 

 which occur outside such a tube have been discovered. In 1S93, 

 Lenard, using a tube provided with a window made of a very 

 hin plate of aluminium, found that a screen impregnated with 

 a soluti<m of a phosphorescent substance became luminous if 

 placed outside the tube in the prolongation of the line from the 

 kathode through the aluminium window. He also found that 

 photographic plates placed outside the tube in this line were 

 affected, and electrified bodies were discharged ; he also ob- 



NO. 1403, VOL. 54] 



tained by these rays photographs through plates of aluminium 

 or quartz. He found that the rays were affected by a inagnet, and 

 regarded them as the prolongations of the kathode rays. This 

 discovery was at the end of last year followed by that of Kontgen, 

 who found that the region round the discharge tube is traversed by 

 rays which affect a photographic plate after passing through sub- 

 stances such as aluminium or cardboard, which are opaque to or- 

 dinary light ; which pass from one substance to another, without 

 any refraction, and with but little regular reflection ; and w hich are 

 not affected by a magnet. We may, I think, for the ]iurposes 

 of di.scussion, conveniently divide the rays occurring in or near a 

 vacuum tube traversed by an electric current into three classes, 

 without thereby implying that they are necessarily distinctly 

 different in physical character. We have ( I ) the kathode rays 

 inside the tube, which are deflected by a magnet ; (2) the Lenard 

 rays outside the tube, which are also deflected by a magnet ; and 

 (3) the Riintgen rays, which are not, as far as is known, deflected 

 by a magnet. Two views are held as to the nature of the 

 kathode rays ; one view is, that they are particles of gas carrying 

 charges of negative electricity, and moving with great velocities 

 which they have acquired as they travelled through the intense 

 electric field which exists in the neighbourhood of the negative 

 electrode. The phosphorescence of the glass is on this view 

 produced by the impact of these rapidly mo\ ing charged particles, 

 though whether it is produced by the mechanical violence of the 

 impact, or whether it is due to an electro-magnetic impulse 

 produced by the sudden reversal of the velocity of the negatively 

 charged particle — whether, in fact, it is due to mechanical or 

 electrical causes, is an open question. This view of the con- 

 stitution of the kathode rays explains in a simple way the 

 deflection of those rays in a magnetic field, and it has lately 

 received strong confirmation from the results of an experiment 

 made by Perrin. Perrin placed inside the exhausted tube a 

 cylindrical metal vessel with a small hole in it, and connected 

 this cylinder with the leaves of a gold-leaf electroscope. The 

 kathode rays could, by means of a magnet, be guided so 

 as either to pass into the cylinder through the aperture, or 

 turned quite away from it. Perrin found that when the kathode 

 rays passed into the cylinder the gold leaf of the electro- 

 scope diverged, and had a negative charge, showing that the 

 bundle of kathode rays enclosed by the cylinder had a charge 

 of negative electricity. Crookes had many years ago exposed a 

 disc connected with a gold-leaf electroscope to the bombardment 

 of the kathode rays, and found that the disc received a slight /Oif/ir'f 

 charge ; with this arrangement, however, the charged particles 

 had to give up their charges to the disc if the gold leaves of the 

 electroscope were to be affected, and we know that it is extremely 

 difficult, if not impossible, to get electricity out of a charged 

 gas merely by bringing the gas in contact with a metal. Lord 

 Kelvin's electric strainers are an example of this. It is a 

 feature of Perrin's experiment that since it acts by induction, 

 the indications of the electroscope are independent of the com- 

 munication of the charges of electricity from the gas to the 

 cylinder, and since the kathode rays fall on the inside of the 

 cylinder, the electroscope would not be affected, even if there 

 were such an effect as is produced when ultra-violet light falls 

 upon the surface of an electro-negative metal when the metal 

 acquires a positive charge. Since any such process cannot 

 affect the total amount of electricity inside the cylinder, it will 

 not affect the gold leaves of the electroscope : in fact, Perrin's 

 experiments prove that the kathode rays carry a charge of 

 negative electricity. 



The other view held as to the constitution of the kathode 

 rays is that they are waves in the ether. It would seein difficult 

 to account for the result of Perrin's experiment on this view, and 

 also I think very difficult to account for the magnetic deflection 

 of the rays. Let us take the case of a uniform magnetic field : 

 the experiments which have been made on the magnetic deflec- 

 tion of these rays seem to make it clear that in a magnetic field 

 which is sensibly uniform, the path of these rays is curved ; 

 now if these rays were due to ether waves, the curvature of the 

 path would show that the velocity of propagation of these waves 

 varied from point to point of the path. That is, the velocity 

 of propagation of these waves is not only affected by the 

 magnetic field, it is affected differently at difterent parts of the 

 field. But in a uniform field what is there to diflerentiate one 

 part from another, so as to account for the variability of the 

 velocity of wave propagation in such a field ? The cur- 

 vature of the path in a uniform field could not be .iccounted 

 for by supposing that the velocity of this wave motion 



