On the General Motion of Long Waves. 



349 



in focus. It is then advanced over the flame until nearly all 

 the light is cut off. On lowering the microscope-tube until 

 the object is once more in focus, and carefully advancing 

 the brass strip until practically all of the flame-image is cut 

 off, we shall find that suddenly the glass particles appear with 

 most astonishing sharpness, every irregularity, angle, and 

 facet showing as distinctly as if we were dealing with great 

 lumps of glass in air. The two photographs accompanying 

 this paper (PI. III.) were taken of the same "field" of glass 

 fragments in oil — one with ordinary illumination, the other 

 by the Schlieren-Methode. It is important to choose a lens 

 of such focus that its focal plane shall be half a millimetre 

 or so below the objective when the microscope is focussed 

 on the object. The scheme is shown in the diagram. 



Object sl/oe 

 Stac£ 



,£B» 



While I doubt if this method will be found to have a very 

 wide application, it appears to me that in certain special 

 cases, perhaps in the examination of rock-sections, it may 

 prove useful. From the nature of things it cannot very 

 well be used with high powers, at least it is not so sensitive 

 under such circumstances. 



XXXIY. The General Motion of Long Waves, icith an Exa- 

 mination of the Direct Reflexion of the Solitary Wave. By 



E. F. GWYTHEK, MA* 



ALTHOUGH most of the interesting cases of fluid moiion 

 are of the steadily progressive type, there are many 

 cases worthy of consideration in which the conditions of pro- 

 gressive motion are not satisfied ; and it is here proposed to 

 deal with the case of long waves to a first approximation in the 

 most general manner. 



* Communicated by the Author. 

 Phil, Mag. S. 5. Vol. 50. No. 304. Sept. 1900. 2 B 



