April 17, 1902] 



NA TURE 



575 



THE PHOTOGRAPHY OF DISTURBANCES IN 



AIR. 

 TN a paper read before the Royal Philosophical Society of 

 Glasgow on December 4, 1901, and published in the 

 Proceeditigs of the Society, Mr. H. S. Allen, of the Blythswood 

 Laboratory, gives an account of " The Photography of Sound 

 Waves and other Disturbances in Air." The method of stria' 

 (Schlieren Methode) was devised by Toepler more than thirty 

 years ago. This method makes it possible by suitable optical 

 arrangements to render visible disturbances in which the refrac- 

 tive index differs but little from that of air. 



One form of these arrangements is shown in the diagram. 

 Fig. I. The light proceeds from a source L which is as nearly 

 as possible a straight line. In the figure this line of light is 

 seen only in section — it is supposed to be at right angles to the 

 plane of the paper. The light issuing from this source falls on 

 a large concave mirror, M, by which it is brought to a focus just 

 in front of the lens of the camera at I. One half of the lens is 

 covered with an opaque screen, having a straight edge parallel 

 to the image of the source, and the apparatus is arranged so 

 that the image falls exactly on this straight edge Then, if all 

 the adjustments are ideally perfect, no light at all will enter the 

 camera so long as the medium through which the light passes 

 is homogeneous. But supposing there is a region in the path of 

 the light having a density different from that of the surrounding 

 atmosphere, some of the light may be bent aside so as to enter 

 the lens of the camera. Such a region is represented in section 

 by the circle in the figure. It is supposed to be of greater density 

 than the air around. The paths of the rays which have been 

 refracted in passing through it are represented by the dotted 

 lines. It will be seen that light traversing the lower portion is 

 bent upwards and enters the camera, while light passing through 

 the upper portion is bent downwards and falls still further than 

 before from the boundary of the opaque screen. If the camera 

 is focussed on this region of greater density, the lower part (that 



is the upper in the camera, the image being reversed) will be 

 illuminated, while the upper portion remains dark. 



In working the method, the source of light and its image on 

 the diaphragm must necessarily be of finite width, and the 

 adjustments are made so that a certain fraction of the width of 

 the image falls on the screen while the light from the remaining 

 portion passes through the lens and gives rise to a uniform field. 

 In these circumstances, the upper part of the region of greater 

 density would appear dark against a light field. The sensitive- 

 ness of the method depends on the relative proportion of the 

 light stopped by the screen and the light that enters the lens. 

 For photographic purposes there must be a moderate amount of 

 light to produce any effect even with the most sensitive plates, 

 so that eye observations are considerably more sensitive. When 

 it is desired to view the disturbances directly the camera is 

 replaced by a telescope, or the image formed by the camera lens 

 is examined by a suitable eyepiece.' 



The mirror used was originally designed for a reflecting 

 telescope. Its diameter was 18 inches,- and it had a radius of 

 curvature of 30 feet 3 inches. 



One of the most striking applications of the method is the 

 photography of sound waves — waves of compression set up by 



1 A somewhat curious effect is observed with the optical arrangements 

 just described which might form the basis of an optical illusion. If the eye 

 is placed close behind the back of the camera (the ground glass screen 

 being removed), the source of light with ihe apparatus for producing the 

 light is distinctly seen, but when an eyepiece focussed on the back of the 

 camera is employed, the apparatus for producing the disturbances in the air 

 is seen with the mirror as a background. In the former case, the eye sees 

 the real image of the source just in front of the lens and so close to it as to 

 be practically unaffected by it, while the image which can be seen with the 

 aid of the eyepiece is so near the eye as to be invisible. 



NO. 1694, VOL. 65] 



sudden electric discharges. Prof. R. W. Wood has taken a large 

 number of photographs showing the behaviour of these waves 

 (Phi!. Mag. xlviii. p. 21S, 1. p^ 148). -' « 



The arrangement of the apparatus is shown in Fig. 2. At the 

 lower part of the diagram are the terminals, which supply an 

 electric current at a high potential. The source of the current 



may be either an induction coil or an influence machine. From 

 one terminal a wire is led to the spark gap placed in the path of 

 the light travelling from the mirror to the camera. It is this 

 spark which gives rise to the wave of compression to be observed, 

 for convenience it may be termed the sound spark. The ter- 

 minals are brass balls | inch in diameter, and they are placed one 

 behind the other, so that the light from the spark may not enter 

 the camera. From this spark gap a wire is led to a second, 

 which serves as the source of illumination, and is therefore pro- 

 vided with magnesium terminals. The circuit is completed by a 

 wire from this point to the second terminal of the electrical 

 machine. It is necessary that the light spark should take place 

 somewhat later than the sound spark, in order to give the sound 

 wave time to travel a sufficient distance from the terminals to be 

 observed. To effect this a condenser is placed in par.allel with 

 the light spark, so that the light spark is delayed by the time 

 necessary to raise the potential of the condenser high enough to 

 spark across the gap. 



A number of photographs were taken illustrating the reflection 

 of a sound wave at surfaces of various forms and the effect of a 

 diffraction grating. The original negatives were I inch in 

 diameter. They were enlarged to about three times this diameter 

 for use as lantern slides. 



The compre.ssion in one of these waves must be considerable 

 compared with that due to an ordinary musical note. We may, 

 perhaps, form a rough estimate of the amount of compression 

 from the fact that the wave-fronts are seen at least as clearly as 



the jets of carbonic acid gas which are described later. The 

 refractive index of carbon dioxide is i '000454, while that of air 

 is I '000294. Let us assume that the density of the compressed 

 air is the same as that of carbon dioxide. According to the 

 law of Gladstone and Dale, the ratio of the densities is the same 

 as the ratio of the refractive indices less unity, that is, in this 



