Objective Reality of Combination Tones. 349 



operation several times. The disturbances were only pro- 

 duced when the pitch was nearly, or very nearly, correct. 

 As soon as a few beats per second were heard the bands 

 became visible. 



Having thus described the various tests which were applied 

 to the apparatus, we may proceed to describe the experiments 

 without dwelling further on the necessary precautions. It is 

 sufficient to say once for all that they were adopted in each 

 case. 



The main object of the enquiry was to obtain evidence as 

 to the objective reality of the combination-tones, and for this 

 purpose the following experiments were arranged. 



Each box of the siren had four circles of holes which could 

 be used separately or together. The number of openings in 

 the upper box were 9, 12, 15, and 16, and in the lower 8, 10, 

 12, and 18. It will be convenient to refer to these as the 

 9 row of holes, and so on. 



Experiment I. 



The 12 and 15 rows of holes in the upper box were opened, 

 and the pitch was raised until the upper note gave slow beats 

 with a fork of 320 vibrations per second. The lower note 

 was then the C of 256 vibrations. The difference or beat tone 

 of 64 vibrations affected the instrument powerfully. The ex- 

 periment was tried both by night and by day. It was diffi- 

 cult to keep the siren exactly at the true pitch, but when the 

 beats were very slow the bands continually disappeared, 

 sometimes for many seconds at a time, then appeared for a 

 moment and then disappeared again. As soon as the pitch 

 was lost by a few beats per second, the bands remained steady 

 and clearly visible. 



The experiment was repeated with the 9 and 12 rows of 

 holes. When the upper note was of 256 vibrations, the 

 lower note was 192 vibrations. The difference-tone of 64 

 vibrations affected the fork very powerfully. 



The experiment was also modified by opening the 10 and 

 12 rows of holes. When the notes corresponded to 320 and 

 384 vibrations respectively, the bands disappeared as before. 



Experiment II. 



In experiment I. the frequencies of the difference-tone and 

 of Konig's first lower beat-tone were identical. The experi- 

 ment was therefore varied by using 8 and 18 rows of holes. 

 The frequency of the difference-tone was thus proportional 

 to 10, while that of Konig's lower beat- tone would be 

 18 — 2x8^=2, When the siren was revolving at the rate of 



