5i8 



NA TURE 



{April 2, 1885 



which will continue increasing their distance from each other, 

 proportionately with the increase of the velocity. If one suc- 

 ceeds in determining micrometrically the distance of one of these 

 lines from the original aperture, he will measure no longer the 

 single deviation of Foucault, but as high a multiple of that 

 deviation as may be desired. The distance of my two mirrors 

 from each other being 5 m., and the velocity of the rotation of 

 the mirror only 200 revolutions per second, the deviation will be 

 five-eighths of that obtained by Foucault — i.e. five-eighths of 

 07 mm., or nearly C44 mm. The tenth image will conse- 

 quently be at 4/4 mm. from the aperture. 



To assure myself, above everything, of the existence of these 

 multiple images, I employed Foucault's mode of observation, 

 and placed before the luminous aperture, at a little distance from 

 the fixed mirror, a plate of glass with parallel faces, inclined at 

 an angle of 45 to the direction of the axis of the mirror. I!y 

 this means there is thrown back laterally a portion sufficiently 

 faint, it is true, but still a portion, of each of the deviated pen- 

 cils which one receives in a microscope. There will then be 

 seen, so soon as the velocity of rotation is great enough to give a 

 continuous image, appearing on the rim of the image of the 

 aperture a second, less distinct image, next a third at the rim 

 of the second, increasing in breadth in proportion as the first is 

 more and more deviated, and ending by separating from one 

 another. With the electric light generated by a small gas- 

 machine of half-horse power, or with the wan sun of these late 

 days, I have managed to see as many as three images and catch 

 a glimpse of the fourth. The actual result very well corre- 

 sponded with my antici] ations. What remains to be done is to 

 improve the method of observation and increase the quantity of 

 light.' 



Suppose the tenth image is sufficiently intense to be observed, 

 I cut away the silver of the mirror from a little rectangle 

 with rims parallel to those of the aperture. The tenth imam' 

 will come to be formed in this rectangle, all the following 

 images will be suppressed, and the deviated pencil traversing 

 the glass of the minor, the posterior face of which is plain 

 and polished, will be received behind on a prism of total 

 reflection, which will transmit it into the micrometric micro- 

 scope. The distance of the rim of the image from the rim 

 of the rectangle will be measured ; then, by an independent 

 operation, the distance of this rim from that of the aperture ; 

 the sum of the two will give the line of magnitude of the devia- 

 tion. It remains to ascertain the order m of this deviation. 

 For this purpose the rotation of the mirror will be accelerated 

 till the image of the order m - 1 comes to be substituted for that 

 which was observed. Let n and 11' represent the numbers of 

 revolutions of the mirror per second, 8 and 5' the lineal values 

 of the simple corresponding deviations, then 



5 = k 11, S' - ); 11 and m 5 = (rn - 1) 5', 



thence 



in 11 = (111 - 1) 11. 

 whence 



u' - 11 



The number of revolutions is measured electrically by the 

 methods M. Cornu has so carefully discussed in his work on the 

 velocity of light : I need not dwell on it. Finally, the measure- 

 ment of the passage of the light is easily got at ; it is that of 

 the distance from each other of the centres of the surfaces of the 

 two mirrors. 



In order to observe a deviation of a rather high order, all that 

 is needed is a sufficiency of light. Now, in this case, 1 manag 

 to augment considerably the proportions of utilised light. In 

 the first place the rotating minor may be made to reflect on its 

 two faces, care being only taken that both have exactly the same 

 radius of curvature. In the second place, having suppressed 

 every object-glass, I am able to utilise the pencil reflected by the 

 rotating mirror throughout the whole space in which it gives a 

 good image of the aperture, and this space is considerable, 

 because the astigmatism resulting from the obliquity does not 

 sensibly affect the rectilineal form of this image. It is, next, 

 possible to tack on to the mirror of 0'20 m , of which I have 

 ' The quantity of light utilised by this mode of observation on a glass is 

 hardly the tenth uf the actual quantity. The ratio of the geometrically 

 decreasing progression representing the intensities of the successive images 

 being 0-656, allowing o'oo for the reflecting power of the silver, the bright- 

 ness of the third image seen by reflection from a plate of glass is inferior to 

 that of the eighth image seen directly. The reflecting power of new silver 

 being 096, it would be possible by its means to attain to t 



spoken, a series of other identical mirrors placed at the same 

 distance in the plane of rotation of the pencil. The condition of 

 identity of the radius of curvature is, besides, much less rigon us 

 for these mirrors than in the case of the two faces of the rotating 

 mirror. It is, however, always indispensable that the movable 

 image given by this latter is reflected exactly on to the surface ol 

 each of the fixed mirrors. 



I have also to remark that it is necessary that the lineal dis- 

 tance of the image observed from the aperture is large enough 

 to allow the observation to be made. For in the thickness of 

 the glass of the mirror and on its two surfaces there will inc. it 

 ably arise a diffusion, as also reflections of the incident light, 

 to embarrass and even frustrate the exact vision of the deviated 

 image when it is too near the aperture. I have just shown that 

 the actual apparatus ought, under good conditions, to show- an 

 image of the sixteenth order, perhaps even one as high as the 

 twentieth — that is to say, at 8'8 m. from the aperture. It would 

 be useful, however, to have recourse to an apparatus of more 

 considerable proportions. 



If 20 m. is taken for the radius of curvature of the minors 

 and for the length of the simple passage of the light, the 

 movable mirror ought to have a diameter cf 0'2O rn. Let there 

 be impressed on it a velocity of rotation of only fifty revolutions 

 per second, the deviation calculated according to the experiment 

 of Foucault will be : — 



4° 



X — = 1*75 mm. 

 400 



The displacement of the twentieth image will then be 35 mm . 

 which, measured to the hundredth of a millimetre, will give an 



approximation of . Now I do not think it impossible to 



3500 

 turn a mirror of o'2o m. as many as fifty revolutions per second 

 without causing deformation of its surfaces. The turbines and 

 the movable pieces of the dynamo-electric machines of the pre- 

 sent day frequently attain a similar velocity. 



It is my duty to make known to the Academy that the 

 necessary for my first and long experiments were genero 

 supplied to me by M. de Romilly, to whom I am hap]'/ 

 make this public testimony of my gratitude. 



ACCIDENTAL EXPLOSIONS PRODUCED 

 NON-EXPLOSIVE LIQUIDS'" 



III. 

 ""THE only real danger which may attend the use of the little 



sponge lamps arises from accidental spilling of spirit used '■ 

 filling them in the neighbourhood of a flame, or from car: 

 out the operation of filling in the vicinity of a light. In 

 such casualties as have been attendant upon the use of petrol, 

 spirit as an illuminant have teen mainly connected with the 

 keeping and handling of the supplies of this very volatile liquid, 

 and are largely attributable to want of caution or to forget ful- 

 ness. The salutary regulation prescribed by law, that v 

 containing the spirit shall bear a conspicuous label indicatin 

 dangerous character, has uncL ubtedly operated very bench' 

 in diminishing the frequency of accidents with it, by constantly 

 admonishing to caution. It is a matter for much surprise and 

 regret that the manufacturers of a class Of miners' safety lain 1 , 



ting of modifications of well-known types, with 

 ordinary oil lamp replaced by the sponge lamp, in which 

 petroleum-spirit is burned, should have allowed trade inti 

 to induce them to mislead those who these lamps will: e- 



gard to the nature of the illuminant supplied with them, by 

 devising a name for it which gives a false indication of its nal 

 being designed to create the belief that it is an artie'e of sp 

 manufacture, allied ia character to a comparatively very sate li] 

 largely used in miners' lamps, while in reality it is a well-known 

 article of commerce, the safe storage and use of which den 

 special precautions and vigilance. 



The lecturer took occasion to point out here, ten years 

 that a large proportion of the accidents arising out of the em- 

 ployment (f petroleum- or paraffin-lamps were not actually d 11 

 to the occurrence of explosions. Thus the incautious can 

 of a lamp, whereby the liquid is brought into contact with ' lie 

 warm portion of the lamp close to the burner, may give rise to i 

 liberation of vapour which, in escaping from the lamp, may bi 



1 Address delivered at the Rnvrd Instmili n '^ Great Britain, Fri 

 March r 3 , 1885, by Sir Fredericl ! el I B, D.C.L., F.R.S., M R 1 

 Continued from p. 496. 



