July 19, 1877] 



NA rURE 



229 



THE VELOCITY OF LIGHT' 



THE correct determination of the velocity of light is a 

 result on which so much in physical science depends 

 that there is good reason for us to give a description of 

 the details of the apparatus used for the purpose of ob- 

 taining the exact value. Until the time that M. Cornu 

 undertook experiments with this object in view the gene- 

 rally received value of the velocity in question was 298,000 

 kilometres per second. This depended on the experi- 

 ments of M. Foucault, who used a rotating mirror on 

 which the rays of light from cross-wires fell, and while 

 the minor was in a certain position were reflected by it 

 to a concave mirror at a distance of 13 J feet, having the 

 revolving mirror at its centre of curvature and so fixed as 

 to return the rays of light to the latter, which reflected 

 them to the point of departure. While however the 

 rotating mirror was in rapid motion, a ray of light re- 

 flected by it to the distant mirror and back from it, 

 would not, unless the passage of light were instantaneous, 

 reach the rotating mirror until the latter had moved from 

 its position of first reflection, and would not therefore 

 return to the point of departure, but to some point near 

 it, depending on the angle through which the rotating 

 mirror had moved in the time between its reflecting the 

 ray to the concave mirror and the return of that ray. By 

 placing the cross-wires in the principal focus of a convex 

 lens the rays proceed in a parallel beam, and on returning 

 form an image of the wires, removed from the wires them- 

 selves, a distance depending on the angular velocity of 

 the mirror and the velocity of light. The cross-wires and 

 their images are rendered visible by viewing them by 

 means of a diagonal reflector of plain glass in front, which 

 at the same time allows sufficient light to pass through to 

 illuminate them.'-* 



In 1S49 iVI. Fizeau devised a method differing from 

 that just described by which he measured the time a ray 

 took to travel from Suresnes to Montmartre and back. 

 The apparatus consisted of a toothed wheel which could 

 be rotated with a known velocity, and having the teeth 

 and intervals equal in size. A pencil of rays was sent 

 through the interval between two teeth to a reflector 

 at Montmartre 28,334 feet distant, which caused the 

 ray to return on itself. So long as the wheel is at rest 

 and the rays pass through an interval, they will be returned 

 through that same interval, but when the wheel turns with 

 sufficient velocity a tooth takes the place of an interval 

 before the ray has time to return from Montmartre and 

 get through, and is therefore interrupted. Uy rotating this 

 wheel faster the next interval will take the place of the 

 preceding one on the return of the ray, which will again 

 get through, and so on passing and being interrupted as 

 the velocity of rotation increases. 



It is obvious, then, that if we know the number of teeth 

 on the wheel and the number of turns per second, say at 

 the instant of reappearance of the spot of light after a dis- 

 appearance, we shall know the interval between the pass- 

 ing away of the ray by the edge of one tooth to its return 

 by the correspondmg edge of the next ; and this is the time 

 the ray has taken to traverse the distance to the reflecting 

 station and back, and from this the velocity of light follows. 

 From these experiments M. Fizeau obtained a velocity 

 of 315,000 kilometres = 196.000 miles per second. 



At that time the velocity of light deduced from the 

 observations of eclipses of Jupiter's satellites, combined 

 with the then accepted solar parallax, was 190,000 miles 

 per second, cloiely agreeing with M. Fizeau's result ; 

 1 Iter determinations of solar parallax have given a smaller 

 result than former ones, and consequently the velocity 



' " Determination de la Vitesse de ]a Lumiere d'apres des Expe'riences 

 executes en 1874. en're I'Observatoire et Montlhery." Par M. A. Cornu. 

 (Par.s: Gauthier V.llars, 1876) 



= From the experiments of M Foucault in iS52 a velocity of 298,000 kilo- 

 metres = 185,157 miles per second was deduced. 



of light deduced therefrom becomes reduced, which again 

 closely agrees with M. Foucault's direct determination. 



In the year 1874 Ae Council of the Paris Observatory, 

 on the proposition of M. Leverrier and M. Fizeau, de- 

 cided on the execution of experiments for the direct deter- 

 mination of the velocity of light, and offered the use of 

 the scientific apparatus at the observatory for the purpose, 

 together with funds for the construction of the necessary 

 instruments To M. Cornu was entrusted the execution of 

 the operations ; and after due consideration and experi- 

 mental comparison, he adopted the method of M. Fizeau 

 in preference to the revolving mirror of M. Foucault. A 

 preliminary series of experiments were carried out in 1871 

 and 1872, between the Ecole Polytechnique and Mont 

 Valdrien, a distance of 10,310 meters, giving a result of 

 185,370 miles per second as the velocity of light, with a 

 probable error of less than i^ij. M. Cornu then com- 

 menced more careful experiments between the Observatory 

 and the Tour de Montlhery, a distance of 22,910 metres. 

 The principle of M. Cornu's arrangement we have 

 already described, it being the same as that of M. 

 Fizeau, but the details of the apparatus are somewhat 

 elaborate, and in his Memoir occupy seven large sheets of 

 plates ; we can, however, to a certain extent describe them. 

 Rays of light from a highly luminous source issuing from 

 a small hole in a diaphragm, pass through a convex lens, 

 and after reflection at an angle of 45 from the surface 

 of a plain piece of glass, are brought to a focus at the 

 circumference of the toothed wheel; the light then traverses 

 an object-glass of fourteen inches diameter, and the 

 parallel rays travel to the reflecting station ; here they are 

 received by an object-glass of six-inch aperture, and about 

 six feet focal length, a: the principal focus of which is a 

 reflecting mirror of silvered glass. From this mirror the 

 rays are returned to the toothed wheel, where an image 

 is formed coinciding with the original image of the hole 

 in the illuminated diaphragm, the rays if not inter- 

 cepted by a tooth, pass onwards, and the greater part of 

 them traverse the diagonal reflector of plain glass 

 and an eyepiece beyond, through which the image 

 formed by reflection from the distant station is viewed. 

 So far we have given an outline of the optical part of the 

 apparatus as well as we can without the use of the drawings 

 by which the details can only be made intelligible. We 

 next come to the toothed wheel, and here certain condi- 

 tions must bs fulfilled : first a velocity of rotation must 

 be obtained capable of admitting a considerable number 

 of orders of extinction ; secondly, the motive power must 

 be such that the observer can easily control the velocity 

 of the wheel ; thirdly, there must be a means of recording 

 the velocity at any instant of time. The motive power is 

 a weight which drives a train of wheels which rotate the 

 toothed wheel, the latter being constructed of aluminium 

 from -^ to yV millimetre in thickness ; wheels of different 

 diameters were used varying from 35 to 48 millimetres. 

 The velocity is recorded on the surface of smoked paper 

 on a roller of about one metre in circumference and half 

 a metre in length, turning on a horizontal axis. The 

 records are made by the action of electro-magnets on 

 light tracers pressing against the surface of the smoked 

 paper. The velocity of the cyHnder carrying the paper 

 is such that a line 20 mm. is traced in a second, and 

 during each revolution the tracers are moved on hori- 

 zontally 15 mm. One of the tracers is put into 

 action at every second by electric communication with 

 a standard clock ; a second is put into action at 

 every yij second by a trembler governed by the pendulum 

 of the clock ; the third moves at each fortieth or four 

 hundredth revolution of the toothed wheel, and the fourth 

 is under the control of the observer. Each of the four 

 tracers is continually in contact with the smoked surface 

 of the paper, and so long as it is not moved sidewise by 

 the electro-magnet, traces a continuous line round the 

 cylinder, but on the passing of a current round the 



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