8o 



NATURE 



[May 24. 1900 



developer, and the amount chanced to be correct. All photo- 

 graphy is done with objective and camera. In photographing 

 the sun, the object is some ninety millions of miles off; in 

 photographing a fluid inclusion in quartz, it is the i/i6th of an 

 inch off — a mere question of detail. Most of these scientific 

 photographs are far easier than the simplest everyday landscape. 



A. R. Hunt. 



Comets and Corpuscular Matter. 



Referring to Prof. J. J. Thomson's article on " corpuscles " 

 in your issue of May 10, it occurs to me that the behaviour of 

 corpuscular matter described therein may have some bearing on 

 comelary phenomena. May not the structure of comets to some 

 extent be explained by assuming that their tails are composed 

 of aggregations of negatively charged particles of extremely 

 minute size, answering to the free corpuscular matter as defined 

 by Prof. Thomson, and which to a large degree may be formed 

 by a sort of " corpuscular dissociation," or detachment, taking 

 place in the comet's nucleus when its temperature is elevated 

 upon nearing the sun ? Since Prof. Thomson's experiments in- 

 dicate the presence of negatively charged matter in kathode rays 

 having a much smaller mass than ordinary atoms, there is 

 reason to believe that matter in this state has properties quite 

 apart from matter in a much coarser state of atomic division. 

 Postulating an electrostatic field as existing in interplanetary 

 space, with the sun as a negative centre or source of electro- 

 static radiation, and assuming that a comet's tail is composed of 

 these corpuscles, the gravitational force it may suffer, when in 

 proximity to the sun, would perhaps be very small in comparison 

 with the electrostatic force existing throughout the vast congrega- 

 tion of these extremely minute particles, and thereby account 

 for the repulsion of the tails of comets when they approach the 

 sun. 



The nuclei of comets may be composed of matter in a much 

 coarser state of subdivision, which, though endowed with posi- 

 tive or opposite electricity, is subject to gravitational influences 

 which determine their course in the neighbourhood of the sun. 



While the above is a partial re-statement of existing hy- 

 potheses, it may, I venture to suggest, be of interest in con- 

 nection with Prof. Thomson's remarkable experiments on 

 matter smaller than atoms. F. H. Loring. 



I Champion Grove, Denmark Hill, S.E., May 18. 



frequency ; and if the natural pitch of the plate is made 

 to approximate that of the resonator and tone, the ampli- 

 tude of the plate's vibrations are rapidly multiplied. 

 To make this amplitude a definitely measurable quantity, 

 the sensitive plate carries at its centre a tiny mirror, 

 which forms one of a system of mirrors constituting 

 Michelson's refractometer {Phil. Mag. 1882, xiii. p. 236). 

 A displacement of the little mirror from its position at 

 rest amounting to a half wave length of light will cause 

 a corresponding shifting to one side of the interference 

 bands, so that each dark band will take the position 

 before occupied by the next dark band. The width of 

 the bands njay be so adjusted that a telescope with 

 micrometer eyepiece can easily subdivide each band 

 into a hundred parts. Hence the displacement of the 

 sensitive plate, while a tone is sounding, could be 

 observed with great precision, if the eye could act with 

 sufficient rapidity to mark the oscillation of any one 

 band. 



That, of course, is out of the question. But it is easy 

 to compound this motion of the bands with another 

 motion perpendicular to it (also in the focal plane), and 

 thus to make the displacements visible. To do this, the 

 interference bands are made to stand vertically in the 

 field, and a screen with a narrow, horizontal slit is inter- 

 posed in the line of sight ; consequently the bands 

 during silence appear in the telescope as a narrow, 

 horizontal strip, composed of the bands reduced to 



A 



A NEW INSTRUMENT TO MEASURE AND 

 RECORD SOUNDS.'^ 

 DIRECT, absolute measurement of the intensity of 

 sound at any point in the air must determine in 

 ordinary units, such as kilogram-metres, the energy 

 involved in the condensations and rarefactions of which 

 the propagation of sound consists. But these pulsations 

 follow each other so rapidly, and the amount of energy 

 involved in even the loudest sound is so infinitesimal, 

 that such measurement is attended with considerable 

 difficulty ; so much, indeed, that probably not a half- 

 dozen laboratories in the world have any instrument 

 whatever purporting to make direct, absolute measure- 

 ments of the energy of sound. 



We owe to Helmholtz (" Wissenschaftlische Abhand- 

 lungen," vol. i. p. 378) a mathematical theory by which 

 we can determine the ratio between the energy of the 

 pulsations of a tone just without, and that within a 

 spherical Helmholtz resonator ; to Lord Rayleigh we 

 owe an expression for the energy of sound in terms of 

 the condensation (" Theory of Sound," vol. ii. Sec. 245). 

 Upon these two results this instrument (like Wien's, 

 Wied. Ann. 1898, p. 834) is founded. 



A pure tone is received into a spherical Helmholtz 

 resonator, a portion of the walls of which is replaced by 

 a small, circular, extremely thin glass plate, situated just 

 opposite the mouth of the resonator. The pulsations 

 within force this plate to vibrate with the tone's 



^} '^^f instrument is described somewhat more fully than it is here in the 

 Monthly Weather A-^/V?*/, July 20, 1899, published by the U.S. Depart- 

 ment ot Agriculture. We are indebted to the courtesy of its editor, Prol. 

 Cleveland Abbe, for the accompanying Illustrations. 



NO. 1595, VOL. 62] 



Fig. I. — The refractometer. The resonator has been unscrewed from the 

 supporting bracket, leaving the sensitive plate and tiny mirror in place. 



square spots of dark and light. Now a small lens, 

 forming the object-glass of the telescope, is mounted 

 upon the end of one tine of a tuning fork, electrically 

 driven, and having the pitch of the tune to be measured. 

 During silence, the vertical vibration of the object-glass 

 stretches out the strip of spots into a rectangle of long, 

 vertical bands. But when the tone sounds, these bands 

 arrange themselves diagonally across the same rectanglb, 

 the slope of the bands increasing with the intensity of 

 the tone. 



The micrometer eyepiece can be rotated on its optical 

 axis, and it is provided with a tangent screw for close 

 adjustment. As it is rotated a vernier moves over a 

 graduated arc, so that the angle of the slope (a) may be 

 measured, as well as the height (Q) of the rectangle, the 

 height {p) of the strip, and the width of five double 

 bands. Putting B = Q-<?, and P = the displacement of a 

 band, we have P = Btan a. The intensity of the tone is 

 proportional to P^, which is thus determined in mean 

 wave-lengths of white light. 



Thus far it has been tacitly assumed that the source 

 of tone is at just the right distance from the receiving 

 resonator for the vibrations of the sensitive plate to be 

 in phase with those of the fork carrying the object- 

 glass. But in ordinary work this agreement in phase 



