228 



NA 7 URE 



{July 4, 1889 



markable exactness most of the features above described. 

 In order to explain the vigour and purity of the colour 

 reflected in certain crystals, it is necessary to suppose that 

 there are a considerable number of twin surfaces disposed 

 at approximate equal intervals. At each angle of in- 

 cidence there would be a particular wave-length for which 

 the phases of the several reflections are in agreement. 

 The selection of light of a particular wave-length would 

 thus take place upon the same principle as in diffraction 

 spectra, and might reach a high degree of perfection. 



In illustration of this explanation an acoustical analogue 

 is exhibited. The successive twin planes are imitated by 

 parallel and equi-distant disks of muslin (Figs, i and 2) 

 stretched upon brass rings and mounted (with the aid of 

 three lazy-tongs arrangements), so that there is but one 



degree of freedom to move, and that of such a character 

 as to vary the interval between the disks without disturb- 

 ing their equi-distance and parallelism. 



The source of sound is a bird-call, giving a pure tone of 

 high pitch (inaudible), and the percipient is a high-pressure 

 flame issuing from a burner so oriented that the direct 

 waves are without influence upon the flame (see Nature, 

 xxxviii. 208 ; Proc. Roy. Inst., January 1888). But the waves 

 reflected from the muslin arrive in the effective direction, 

 and if of sufficient intensity induce flaring. The experi- 

 ment consists in showing that the action depends upon 

 the distance between the disks. If the distance be such 

 that the waves reflected from the several disks co-operate,^ 

 the flame flares, but for intermediate adjustments re- 

 covers its equilibrium. For full success it is necessary 



>^- 



Fic I. 



GENERAL SCHEME 



Fic2. 



Detail of Lazy-tc 



©SENSITIVE riAME 



that the reflective power of a single disk be neither too great 

 nor too small. A somewhat open fabric appears suitable. 



It was shown by Brewster that certain natural specimens 

 of Iceland spar are traversed by thin twin strata. A con- 

 vergent beam, reflected at a nearly grazing incidence from 

 the twin planes, depicts upon the screen an arc of light, 

 which is interrupted by a dark spot corresponding to 

 the plane of symmetry. [Shown.] A similar experiment 

 may be made with small rhombs in which twin layers have 

 been developed by mechanical force after the manner of 

 Reusch. 



The light reflected from fiery opals has been shown by 

 Crookes to possess in many cases a high degree of purity, 



rivalling in this respect the reflection from chlorate of 

 potash. The explanation is to be sought in a periodic 

 stratified structure. But the other features differ widely 

 in the two cases. There is here no semicircular evanes- 

 cence, as the specimen is rotated in azimuth. On the 

 contrary, the coloured light transmitted perpendicularly 

 through a thin plate of opal undergoes no change 

 when the gem is turned round in its own plane. This 

 appears to prove that the alternate states are not related 

 to one another as twin crystals. More probably the 

 alternate strata are of air, as in decomposed glass. The 

 brilliancy of opals is said to be readily affected by atmo- 

 spheric conditions. 



NOTES. 



The thirty-eighth meeting of the American Association for the 

 Advancement of Science will be held at Toronto. On Tuesday, 

 August 27, at noon, a meeting of the Council will be held 

 at the Queen's Hotel, where will be the hotel head-quarters of 

 the Association. On Wednesday, August 28, the first general 

 session will begin at 10 o'clock in the forenoon in the Con- 

 vocation Hall, University Buildings. After the adjournment of 

 the general session, the several Sections will organize. In the 

 afternoon the Vice-Presidents will give their addresses before 

 their respective Sections, and in the evening there will be a 

 general session, when the retiring President, Major J. W. Powell, 

 will deliver his address. The sessions will continue until the 

 Tuesday evening following, and on Wednesday morning, Sept- 

 ember 4, a meeting of the Council will be held. Saturday, 

 August 31, will be devoted to excursions. The meeting will 

 close with excursions extending to September 7. The general 

 sessions and the meetings of the Sections will be held in the 

 University Buildings, where also will be the offices of the local 

 committee and of the permanent secretary during the meeting. 



Science states that arrangements have been made for a discussion 

 in Section B on the " Relative Merits of the Dynamometric 

 and Magnetic Methods of obtaining Absolute Measurements of 

 Electric Currents." Prof. Thomas Gray, of the Rose Polytechnic 

 Institute, will open the discussion with a paper on the subject, 

 and he will exhibit one or more of Sir William Thomson's most 

 recent forms of electric balance. Arrangements have been made 

 by the local committee for the proper care and exhibition of 

 instruments and specimens. 



Efforts are being made by the American Association for 

 the Advancement of Science to form a National Chemical 

 Society, with its head-quarters at Washington. A meeting will 

 be held to consider the matter during the session of the 

 American Association at Toronto. 



The annual general meeting of the Marine Biological 

 Association was held in the rooms of the Royal Society on the 

 26th ult. In the absence of Prof. Huxley, the chair was taken 

 by Sir E. Bowman, and there were present, among others, Lord 



' If the reflection were perpendicular, the interval between successive disks 

 would be equal to the half wave-length, or to some multiple of this. 



