1895.] 



on the Physical Work of von Helmholtz. 



487 



For this purpose the rather bulky- 

 apparatus which you see before you has 

 been constructed. I should hardly have 

 been able to realise the idea embodied in 

 it, at all events in time to show it to you 

 this evening, if I had not been favourably 

 situated in two respects. In the first place 

 I have had the zealous co-operation of one 

 of my assistants, Mr. Edwin Edser, who 

 has not only made all the parts of the 

 apparatus that required to be newly made, 

 but has thrown himself into the investiga- 

 tion with the utmost energy, working at it 

 late and early, and making many valuable 

 suggestions and improvements. In our 

 joint work we have been helped by some 

 of my senior students, and notably by 

 Messrs. Cullen and Forsyth. In the 

 second place, I have had at my disposal the 

 magnificent collection of acoustical appa- 

 ratus in the National Museum at South 

 Kensington, some of which I am allowed, by 

 the kindness of the Department of Science 

 and Art, to bring here this evening. 



The essential part of the apparatus 

 is a tuning-fork F, to one prong of 

 which is attached a mirror M, and to the 

 other a square of thin wood, strengthened 

 by ribs, which is of the same weight as 

 the mirror. The fork thus loaded has 

 been compared with one of Konig's large 

 standards by means of Lissajous' figures. 

 Its frequency does not differ from 64 

 complete vibrations per second by more 

 than one vibration in two minutes. The 

 shank is supported by a mass of lead, 

 which in turn is placed upon a paving- 

 stone. Upon this stone also rest the other 

 mirrors necessary for producing Michel- 

 son's interference bands. The mirror M 1 

 is silvered so thinly that half the light 

 which falls upon it is reflected, and half is 

 transmitted. 



A ray proceeding from the lantern L 

 will be divided at M x into two, which fol- 

 low the paths LM^M^ and LM^M^ 



respectively. Interference bands are thus produced which can be pro- 

 jected on to a screen, so as to be rendered visible to a large audience. 



2 l 2 



