July 28, 1 881] 



NATURE 



303 



vibration of the diaphragm occurs capable of producing sonorous 

 effects. It has occurred to me that Mr. Preece's failure to 

 detect with a delicate microphone the sonorous vibrations that 

 were so easily obsei-ved in our experiments might be explained 

 upon the supposition that he had employed the ordinary form of 

 Hughes' microphone shown in Fig. i, and that the vibrating 

 area was confined to the central portion of the disk. Under 

 such circumstances it might easily happen that both the supports 

 (a b) of the microphone might touch portions of the diaphragm 

 which were practically at rest. It would of course be interest- 

 ing to ascertain whether any such localisation of the vibration as 

 that supposed really occurred, and I have great pleasure in 

 showing to you to-night the apparatus by means of which this 

 point has been investigated (see Fig. 2). 



The instrument is a modification of the form of microphone 

 devised in 1827 by the late Sir Charles Wheatstone, and it con- 

 sists essentially of a stiff wire (a), one end of which is rigidly 

 attached to the centre of a metallic diaphragm (b). In Wheat- 

 stone's original arrangement the diaphragm was placed directly 

 against the ear, and the free extremity of the wire was rested 

 against some sounding body, like a watch. In the present 

 arrangement the diaphragm is clamped at the circumference like 

 a telephone-diaphragm, and the sounds are conveyed to the e.ar 

 through a rubber hearing-tube (c). The ^^ ire passes through the 

 perfor.-ited handle (D), and is exposed only at the extremity. 

 When the point (a) was rested against the centre of a diaphragm 

 upon which was focussed an intermittent beam of sunlight, a 

 clear musical tone was perceived by applying the ear to the 

 hearing-tube (c). The surface of the diaphragm was then ex- 

 plored with the point of the microphone, and sounds were 

 obtained in all ])arts of the illuminated area and in the corre- 

 sponding area on the other side of the diaphragm. Outside of 

 this area, on both sides of the diiphragm, the sounds became 

 weaker and weaker, until at a certain distance from the centre 

 they could no longer be perceived. 



At the points wliere one ■\\'ould naturally place the supports of 

 a Hughes micro]ihone (see Fig. l) no sound was observed. We 

 were also unable to detect any audible effects when the point of 

 the microphone was rested against the support to which the dia- 

 phragm was attached. The negative results obtained in Europe 

 by Mr. Preece may therefore be reconciled uith the positive 

 results obtained in America by Mr. Painter and myself. A still 

 more curious demonstration of localisation of vibration occm-red 

 in the case of a large metallic mass. An intermittent beam of 

 sunlight was focussed upon a brass weight (i kilogi'am.) and the 

 surface of the weight was then explored with the microphone 

 sho«n in Fig. 2. A feeble but distinct sound was heard upon 

 touching the surface within the illumiuated area and for a short 

 distance outside, but not in other parts. 



In this experiment, as in the case of the thin diaphragm, abso- 

 lute contact between the point of the microphone and the surface 

 explored was necessary in order to obtain audible effects. Now I 

 do not mean to deny that sound-\\aves may be originated in the 

 manner suggested by Mr. Preece, but I think that our experi- 

 ments have demonstrated that the kind of action described by 

 Lord Rayleigh actually occurs, and that it is sufficient to account 

 for the audible effects observed. 



EXPERIMENTAL DETERMINATION OF THE 

 VELOCITY OF WHITE AND COLOURED 

 LIGHTS 

 'T^HE method employed in this research to measure the velocity 

 of light resembled the method of M. Fizeau, sub--equently 

 employed by M. Cornu. A revolving tootlied wheel is employed 

 in the same way to alter the intensity of the light reflected from 

 a distance. In the present method, however, there are two dis- 

 tant reflectors instead of only one. They are separated by a 

 distance of a quarter of a mile. The observinsj telescope and 

 the two reflectors are almost in the same line. The observer sees 

 two stars of light, which go through their phases with different 

 periods as the toothed wheel is revolved at increasing speeds. 

 One star is increasing, while the other is dimini-hmg, in intensity, 

 with increase of speed of the to. thed wheel. 1 he speed required 

 to produce equality of the light is determined by means of a 

 chronograph. 



By cho. .sing such a speed as gives a maximum of one star at 

 the same speed as a minimum of the other, a pair of observations 



■ Abstract of a paper by Dr. J. Young, F.R.S., and Prof. G. Forbes, read 

 before the Ruyal Society. March 19. 



eliminates all cause of doubt arising from varying brightness in 

 the stars, and ratio of the width of a tooth to the width of a 

 space. The distances were observed by triangulation with the 

 Ordnance .Survey 18-inch theodolite, using as a base hne a side 

 of one of the Ordnance Survey triangles. The source of light 

 was an electric lamp. The velocities (uncorrected for rate of 

 clock, and reduction to a vacuum) measured are as follows : — 



187,707 

 188,405 

 187,676 

 186,457 

 185,788 

 186,495 

 187,003 

 186,190 

 186,830 

 187,266 

 188,110 

 188,079 



Mean 



187,167 miles a second. 



The correction to vacuum is -^ 54 miles a second. The cor- 

 rection for rate of clock to a mean solar time is -f 52 miles a 

 second. 



The final results for the velocity of the light from an electric 

 lamp in vacuo is 187,273 miles a second, or 301,382 kilometres 

 a second. 



Using Struve's constant of aberration 20"445", we obtain for 

 the solar parallax the value S77", and for the mean distance of 

 the sun 93,223,000 miles. 



On February 11, 1881, the reflected stars were seen to be 

 colomed, one reddish, the other bluish. The particular colour 

 of a particular star depended upon the speed of rotation of the 

 toothed wheel. -That star which was increasing with increase of 

 speed of the toothed wheel was reddish, that one that was dimi- 

 nishing with increase of speed was bluish. This seems to be 

 caused by the fact that blue rays travel quicker than red rays. 



A number of tests were made to judge of the accuracy of this 

 conclusion, and they confirmed it. In the final arrangements, 

 the electric light was acted upon by a bisulphide of carbon prism, 

 and part of a pure spectrum was used. Differential measure- 

 ments were then made to find the difference in velocity of rota- 

 tion of the toothed wheel, required to produce equality of red 

 and of blue lights. The most convenient method was to use a 

 driving weight slightly in excess of that required to produce 

 equality of the light, then to fix to the pulley carrying the 

 weights one end of a piece of stout india-rubber tubing, the 

 other end being fixed to a point above. This gradually dimi- 

 nished the effec:ive driving weight. The equality of red lights 

 was first noted, the colour of the light was changed, and the 

 interval of time until the blue lights were equal was measured. 

 The rate at which the india-rubber diminished the speed was 

 afterwards measured by the aid of the chronograph, and thus 

 the difference of speed determined. The mean of thirty-seven 

 determinations in this and other ways gave the result that the 

 difference in velocity between red and blue lights is about I '8 

 per cent, of the whole velocity, blue travelling most rapidly. 



The general conclusion seems to be supported by a comparison 

 of the velocity of light measured by M. Cornu and Mr. Michel- 

 son, where the source of light usually employed is taken' into 

 consideration. These are the only accurate measurements of 

 the velocity of light hitierto published. They give us the 

 following results : — 



Usual Source of Light. ^ a'^Second.' 



Michelsoii's research ... The sun near horizon ... 299,940 



Cornu's ,, ... Lime light 300,400 



The present „ ... Electric light 301,382 



Classifying the sources of light used by Cornu, we get the 

 following approximate relative velocities : — 



_ .: T • V No. of Approximate Relative 



Source of Light. Observations. Velocity. 



Petroleum 20 298,776 kilos. 



Sun near horizon ... 77 300,242 ,, 



Lime light 449 300.290 .. 



All these results seem to support the view that the more re- 

 frangible the source of light, the greater is the velocity. Bu 

 the evidence of the present observations, indicating an excess of 



