290 



Photograjpliy of SouncUvjaves, &c. 



most difficult to prepare, as several reflections have to be considered. 

 It has been completed for three reflections, and Mr. Max Mason, of 

 Madison, to whom I am greatly indebted for his patient work in 

 assisting me, is going on with the series. As will be seen, the wave 

 has already become quite complicated, and it will be interesting to see- 

 what further changes result after three or four more reflections. I am 

 also under obligations to Professor A. B. Porter, of Chicago, who pre- 

 pared the set of drawings illustrating the passage of a wave out from 

 the principal focus of a hemispherical mirror. 



Fig. 11. 



A number of points taken at intervals along the film are here 

 reproduced, and give a fair idea of the transformations. Fig. 9 shows 

 the plane wave entering the hemispherical mirror, while in fig. 10 we 

 have a spherical wave starting on the principal focus of a similar 

 reflecting surface (compare fig. 9 with fig. 2, and fig. 10 with fig. 4), 

 Fig. 11 shows the evolutions of the wave shut upon the complete 

 spherical mirror, and shows the development of the complicated photo- 

 graphed forms shown in fig. 6. 



