Photography of Sound- Waves by the u Scalier en- Methode J" 219 



quality obtainable, and of rather long focus, is the most 

 important part of the device. I have been using the object- 

 glass of a small telescope figured by the late Alvan Clarke. 

 Its diameter is five inches, and the focal length about six feet.' 

 I have no doubt but that a smaller lens could be used for 

 viewing the waves, but one of at least this size is desirable 

 for photographing them. 



This lens is mounted in front of a suitable source of light 

 (in the present case an electric spark), which should be at 

 such a distance that its image on the other side of the lens is 

 at a distance of about fifteen feet. 



The image of the spark, which we will suppose to be" 

 straight, horizontal, and very narrow, is about two-thirds 

 covered with a horizontal diaphragm (a), and immediately 

 behind this is placed the viewing-telescope. On looking into 

 the telescope we see the field of the lens uniformly illumi- 

 nated by the light that passes under the diaphragm, since 

 every part of the image of the spark receives light from the 

 whole lens. If the diaphragm bo lowered the field will 

 darken, if it be raised the illumination will be increased. In 

 general it is best to have the diaphragm so adjusted that the 

 lens is quite feebly illuminated, though this is not true for 

 photographic work. Let us now suppose that there is a 

 globular mass of air in front of the lens of slightly greater 

 optical density than the surrounding air (b). The rays of 

 light going through the upper portion of this denser mass 

 will be bent down, and will form an image of the spark 

 below the diaphragm, allowing more light to enter the 

 telescope from this particular part of the field ; consequently, 

 on looking into the instrument, we shall see the upper 

 portion of the globular mass of air brighter than the rest of 

 the field. The rays which traverse the under part of a b," 

 however, will be bent up on the contrary, forming an image 

 of the spark higher up, and wholly covered by the diaphragm, 

 consequently this part of the field will appear black. It will 

 readily be understood, that with the long path between the 

 lens and the image a very slight change in the optical density 

 of any portion of the medium in front of the lens will be 

 sufficient to raise or depress the image above or below the 

 edge of the diaphragm, and will consequently make itself 

 manifest in the telescope. 



The importance of using a lens of first class quality is 

 quite apparent, since variations in the density of the glass of 

 the lens will act in the same way as variations in the density 

 of the medium before it, and produce unequal illumination of 

 the field. It is impossible to find a lens which will give an • 



