426 



NA TURE 



\_March 28, 1878 



to all his projects. There can be no doubt that Angola, 

 to the elucidation of the natural history of which Mr. 

 Monteiro contributed so largely, still presents a fine field 

 for the collector, and it is to be hoped that some one will 

 be found who will continue the researches so well insti- 

 tuted by the deceased traveller. 



SOUND COLOUR-FIGURES 



'X*HE great interest excited by Prof. Bell's telephone 

 ■■• and Mr. Eddison's phonograph, in which an elastic 

 disc or membrane faithfully takes up the highly complex 

 vibrations due to sounds of the human voice, has directed 

 renewed attention to the optical methods hitherto employed 

 in studying the motion of resonant media. These have, 

 in important instances, been based on observations of the 

 secondary effects produced by sonorously vibrating 

 bodies. Thus Chladni watched the behaviour of sand 

 strewn upon sounding plates and membranes ; Konig 

 that of gas flames acted on by aerial vibrations. The 

 present article describes an analogous method depending 

 on the colours reflected from slightly viscous liquid films 

 when thrown into sonorous vibration. 



The ordinary phenomena called the " colours of thin 

 plates " are sufficiently well known, but a short description 

 of them, taken from a standard work on Physical Optics, 

 may still not be out of place here as a reminder. 



" If the mouth of a wine-glass be dipped in water, which 

 has been rendered somewhat viscid by the mixture of 

 soap, the aqueous film which remains in contact with it 

 after emersion will display the whole succession of these 

 phenomena. When held in a vertical plane, it will at 

 first appear uniformly white over its entire surface ; but, 

 as it grows thinner by the descent of the fluid particles, 

 colours begin to be exhibited at the top, where it is 

 thinnest. These colours arrange themselves in horizontal 

 bands, and become more and more brilliant as the thick- 

 ness diminishes ; until finally, when the thickness is 

 reduced to a certain limit, the upper part of the film 

 becomes completely black. When the bubble has arrived 

 at this stage of tenuity, cohesion is no longer able to 

 resist the other forces which are acting on its particles, 

 and it bursts." — (Lloyd's " Wave-Theory of Light," 

 p. 100.) 



If the film, instead of remaining at rest, is thrown into 

 sonorous vibration, totally distinct colour-phenomena 

 instantly present themselves. A rough idea of their 

 general character may be obtained without the aid of any 

 apparatus as follows. While washing the hands, after 

 getting a good lather, a film can easily be formed between 

 the thumb and forefinger of one hand held in a horizontal 

 plane ; the other hand suppUes an extemporised tube 

 through which a note can be sung, and so vibrations 

 caused to impinge on the lower surface of the film. 



If this is done the reflected colours will be seen to be 

 in regular motion, and, in particular, a number of small 

 eddies of colour will be observed whirling about fixed 

 centres of rotation. Steady coloured bands may also be 

 sometimes recognised, but with much greater difficulty. 



Fixed bands and stationary vortices form, in fact, the 

 constituent elements of all the sound colour-figures obtain- 

 able by film-reflection. 



In order to study these in detail a specially arranged 

 apparatus is, of course, requisite. I have found the 

 following give excellent results. 



An L-shaped cylindrical brass tube is permanently fixed 

 upon a wooden stand, with its two limbs vertical and hori- 

 zontal. The vertical limb terminates in a narrow flat circular 

 ring. The open orifice of the horizontal limb is fitted into 

 a caoutchouc tube of equal bore, ending in a trumpet- 

 shaped mouth- piece. For the purpose of supporting the 

 films operated on, I use a series of metallic discs pierced 

 with apertures of various shapes and sizes. On covering 

 one of these, by means of a camel-hair brush, with some 



weak solution of soap,' a film of considerable durability 

 will be formed upon it. The disc should first be held 

 in a vertical plane until the coloured bands have begun 

 to show themselves, and then laid gently upon the hori- 

 zontal ring prepared for its reception. The observer 

 places himself so as to get a good view of the assem- 

 blage of colours reflected by the film, and the instrument- 

 is ready for use. Sounds of tuning-forks, whistles, organ- 

 pipes, &c., or notes of the human voice have only to be 

 produced near its mouthpiece, in order that their vibra- 

 tions may be conducted to the film, and the resulting 

 phenomena observed. 



The forms thus presented are of endless variety and 

 great beauty. They almost invariably include both 

 motionless curvilinear bands of colour very regularly 

 disposed, and also a system of colour-vortices revolving 

 about fixed nuclei. The contrast between the steady and 

 moving portions of the figures is always very striking, and 

 the effects of changing tint which accompany the progres- 

 sive thinning of the film gorgeous in the extreme. When 

 the moment of its dissolution is close at hand, patches of 

 inky blackness invade the field, until at last there is some- 

 times nothing left but an ebony background, with here 

 and there a few scraps of light, either at rest or still flying 

 round their former orbits, the remnants of fixed bands 

 and whirling vortices. 



That the results obtainable by the mode of experiment- 

 ing above described are likely to present a practically 

 endless variety of form, will be at once obvious from an 

 enumeration of the several causes which may influence 

 the assemblage of colours reflected at a given instant 

 from a given film acted on by the vibrations of a given 

 sound. These are : — i. The shape of the film ; 2. Its 

 size ; 3. Its consistency ; 4. The intensity of the sound ; 5. 

 Its pitch ; 6. Its quality ; 7. The direction in which the 

 sound-vibrations take place with reference to the plane of 

 the film. 



It thus .appears that each colour- figure observed tnay 

 be a function of not less than seven ^ independent 

 variables ; and on experiment this proves to be the fact. 

 An alteration made in any one of these elements, while 

 all the rest are kept constant, produces a corresponding 

 change in the appearances observed. The intensity of 

 the sound does not, it is true, affect the form of the figure, 

 but controls the rate of its vortical motion ; the louder the 

 sound the more rapid the rotation of the colour-whirls. 

 All the other elements act directly on form. 



It is evident from what has preceded that an attempt 

 at anything like a general classification of sound colour- 

 figures would afford materials for a considerable volume. 

 All that can be done within the present narrow limits is 

 to draw attention to a few points of special interest. 



Dependence of Form on Pitch. — This is perhaps most 

 distinctly shown by alternately stroking with a resined 

 bow two mounted tuning-forks of different pitch, the open 

 ends of whose resonance-boxes are placed close to the 

 mouthpiece of the Phoneidoscope. As long as the same 

 aperture is used, and the film kept at one degree of 

 consistency by frequent renewal, each note will instantly 

 call forth its own colour-figure for any number of alter- 

 nations. T^his mode of experimenting has the advantage 

 of giving perfectly steady and sharply defined figures. 

 But the successive alterations of form due to changing 

 pitch are more interestingly shown by singing^ the diatonic 

 or chromatic scale, on some single vowel, into the Phonei- 

 doscope. The complete change of figure consequent on 



' Castile soap, I find, answers extremely well. 



^ It is manufactured and sold under the title of the " Phoneidoscope," by 

 S. C. Tisley and Co., Philosophical Instrument Makers, 172, Brompton 

 Road, S.W. 



3 A reader of Helmholtz will see that I might have added an eighth 

 element by taking into account differences of phase among partial tones, 

 which, though inoperative on quality, directly affect mode of resultant 

 vibration. 



4 A pitch-pipe with a sliding piston may be substituted for the voice in 

 this experiment. 



