156 BULLETIN OF THE 



When a beam of light is brought to a focus by means of a 

 lens, the beam diverging from the focal point becomes weaker as 

 the distance increases in a calculable degree. Hence, if we can 

 determine the distances from the focal point at which two different 

 substances emit sounds of equal intensity, we can calculate their 

 relative sonorous powers. 



Preliminary experiments were made by Mr. Tainter during my 

 absence in Europe to ascertain the distance from the focal point of 

 a lens at which the sound produced by a substance became inau- 

 dible. A few of the results obtained will show the enormous differ- 

 ences existing between the different substances in this respect. 



Distance from Focal Point of Lens at which Sounds became Inaudible 

 with Different Substances. 



Zinc diaphragm, (polished) I-5 1 m - 



Hard rubber diaphragm ^ 1.90 m. 



Tin-foil "■ 2.00 m. 



Telephone " (Japanned iron) 2.15 m. 



Zinc " (unpolished) 2.15 m. 



White silk, (In receiver shown in Fig. I.) 3.10 m. 



White worsted, " " " 4-°! m - 



Yellow worsted, " " " 4.06 m. 



Yellow silk, " " " 4.13 m. 



White cotton-wool, " " " 4.38 m. 



Green silk, " " " 4.52 m. 



Blue worsted, " " " 4.69 m. 



Purple silk, " " " 4.82 m. 



Brown silk, " " " 5.02 m. 



Black silk, " " " 5.21m. 



Red silk, " " " 5.24 m. 



Black worsted, " " " 6.50 m. 



Lamp-black. In this case the limit of audibility could not be deter- 

 mined on account of want of space. 

 Sound perfectly audible at a distance of 10.00 m. 



Mr. Tainter was convinced from these experiments that this field 

 of research promised valuable results, and he at once devised an 

 apparatus for studying the effects, which he described to me upon 

 my return from Europe. The apparatus has since been constructed 

 and I take great pleasure in showing it to you to-day. 



(1.) A beam of light is received by two similar lenses, (A B, 

 Fig. 10,) which brings the light to a focus on either side of the 



