222 Mr. R. W. Wood on Photography of 



wave-front produced by reflexion, refraction, &e. Fig. 1 

 is a contact-print of a small portion of one of the plates, and 

 shows the images exactly as they were obtained on the 

 sensitive film. The other figures are all enlarged, and in 

 general a series of four is given in each case to show the 

 successive positions of the wave-front. Each picture shows 

 the circular field of the telescope-lens with the two brass rods 

 crossing it and supporting in the centre the two balls between 

 which the sound-spark passes. The hot air rising from the 

 spark appears in most of the pictures like a puff of steam 

 above the ball. 



In fig. 2 we have a series illustrating the reflexion of a 

 spherical wave from a flat plate, which theory demands shall 

 be also a sphere with its centre of curvature below the plate. 

 The sound-wave appears as a circle of light and shade sur- 

 rounding the image of the central balls. In No. 3 of the 

 series it will be noticed that the wave-front is gaining a little 

 where it is moving in the warm air above the spark, the 

 front bulging out a little in this place. 



Fig. 3. — Here the wave starts in the focus of a concave 

 reflector, and the transformation of the spherical wave into 

 a plane wave is beautifully shown, the reflected portion, or 

 echo, appearing as an approximately straight line moving up 

 out of the reflector. This series is very useful in illustrating 

 the action of a concave mirror on light emanating from its 

 focus. 



A very beautiful series of eight pictures, illustrating the 

 action of a concave reflector on a wave starting at some 

 distance outside of its focus, is shown in fig. 4. The diverging 

 spherical wave-front after reflexion is seen to be converging, 

 the wave shrinking to a point in the fourth one of the series, 

 the point being the conjugate focus of the reflector. From 

 now on it diverges, and passes out of the reflector, as is 

 shown in the succeeding pictures. 



It is interesting to follow the changes which the wave- 

 front undergoes on entering and leaving the mirror. In 

 No. 1 of the series we have the wave entering the mirror, the 

 portions which have already encountered the steep sides of 

 the reflector and undergone reflexion trailing along behind. 

 In No. 2 the whole wave has undergone reflexion, the central 

 portion is transformed into a converging shell, with which a 

 portion of the two waves reflected from the steep sides 

 appears to unite. As the converging shell shrinks up towards 

 the focus the remnants of the two lateral wave-fronts run in 

 under it and come together just as the main disturbance 

 comes, to focus (Nos. 3 and 4). 



