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SCIENCE. 



[N. S. Vol. XI. No. 276. 



the effect of external influences in modifying 

 vibrations, the former being grouped into such 

 influences as modify pitch and those which en- 

 courage or discourage vibration. Thus in the 

 latter case, an impulse would have to actuate a 

 pendulum while passing through its position of 

 equilibrium ; in the first case the impulse must 

 be applied at either elongation. The principle 

 is illustrated in its bearing on the sounds fre- 

 >quently obtained in glassblowing, on the chem- 

 ical harmonica, and on other similarly subtle 

 methods of sound production. A second paper 

 on absolute pitch accentuates the fact that two 

 equations are given when the frequency-ratio 

 and number of beats per second of two notes of 

 a selected interval are given from which the 

 absolute pitch of both may be computed. The 

 inferences are tested with modifications by 

 aid of the common harmonium. A new series 

 of acoustic experiments deals with the produc- 

 tion of pure tones from sounding flames on 

 suitably modifying the resonator, with Savart's 

 region of silence on reflection, with sensitive 

 flames (which seem to fascinate Rayleigh as 

 they did Tyndall and by which the remarkable 

 investigations on jets above referred to were 

 suggested), etc. Among the results we find 

 that sensitive flames are excited at loops and 

 not at nodes, that Eijke's notes (produced by 

 heated gauze on cooling in a pipe) can be raised 

 to an intensity sufficient to shake a room. 

 Experiments are given on the effect of a barrier 

 in promoting interference between the two 

 halves of an organ pipe. In an ingenious ex- 

 periment in which the chimney is made avail- 

 able as a source of draft, it is shown that the 

 vibrations of the strings of an iEolian harp are 

 at right angles to the direction of the wind. 

 A final series of acoustic observations begins 

 with the full discussion of Mayer's well-known 

 experiment on intermittent sounds. After 

 showing a new form of siren, an experiment is 

 described for obtaining the interferential sound 

 shadow of a circular disc, an analogue of the 

 optical experiment. The last acoustical paper 

 included is an explanation of the photophone. 

 The remaining papers of the volume are 

 largely devoted to optics. We notice in par- 

 ticular a long and frequently quoted paper 

 (1879-80) on the resolving power of telescopes 



with especial reference to spectroscopy.* Start- 

 ing from the deductions of Airy, Verdet and 

 others, Rayleigh computes the visibility curves 

 for single and double lines, single and variously 

 doubled slits. In an examination of the pris- 

 matic spectroscope it appears that the resolving 

 power for a given glass is proportional to the 

 total thickness traversed without regard to the 

 number, angles or settings of the prisms. The 

 aberration errors and the degree of accuracy 

 required in the surfaces are abstrusely treated 

 in detail and a final paragraph is devoted to 

 the designing of the spectroscope. A subse- 

 quent theoretical paper deals with reflection 

 when the transition at the boundary of two 

 media is gradual and not abrupt as usually 

 assumed by the great opticians (Fresnel, Green, 

 Cauchy and others). Passing this and an ex- 

 perimental method (grating) of measuring the 

 resolving power of telescopes, as well as another 

 on the definition of images formed without 

 lenses, we come to Rayleigh' s first considerable 

 papers (1881) on the electromagnetic theory of 

 light. It would appear from this that Rayleigh 

 like Kelvin was late in his acceptance of Max- 

 well's optics, certainly a regrettable circum- 

 stance by which the advance of science was 

 retarded. It is the object of the present long 

 investigation, to find an electromagnetic basis 

 for Fresnel' s optics, particularly in relation to 

 reflection and to double refraction. In differ- 

 ent ways Hemholtz, Lorentz, J. J. Thomson 

 and others have all worked successfully at this 

 problem. It is well known that to explain 

 double refraction Fresnel postulated differ- 

 ences of rigidity of the ether in different direc- 

 tions ; to explain reflection such a change of 

 rigidity in passing from one medium to another 

 is precluded. Neumann and MacCullagh have 

 endeavored to obviate the inconsistency by 

 replacing differences of rigidity by differences 

 of density, but the elastic theory resulting is 

 none the less imperfect. The electromagnetic 

 theory of light based on radically different laws 

 avoids these discrepancies at the outset. Natur- 

 ally in Rayleigh' s work the scattering reflection 

 of moats is particularly considered as a test of 



* In America, as we know, similar work has been 

 remarkably promoted by the researches of Professor 

 Michelson. 



