180 ANNUAL OF SCIENTIFIC DISCOVERY. 



carbonic acid ; this globe was suspended about one foot distance from 

 a common watch ; on placing the ear upon the opposite side it was 

 found that a focus existed where the intensity of the sound was much 

 more remarkable than elsewhere, but he was not able to obtain 

 definite results. 



In a second experiment the globe of gold-beater's skin was replaced 

 by a lens, made of very thin letter paper, it answered no better, and 

 the author abandoned, for a time, his researches. The discovery of 

 collodion, (a solution of gun cotton in ether, which leaves in evapor- 

 ation a very thin, strong, and transparent pellicle, which can be readily 

 separated from the surface on which it is deposited,) determined him 

 to undertake new experiments. On cutting from a very large globe 

 of collodion, two equal segments which were fixed at their bases upon 

 an iron hoop, he fashioned a sort of lens ; or, rather, it was made for 

 him by M. Mullen, of Breslau, so that the interior could be filled with 

 any gas. The iron ring was 307 millimeters in diameter, and 61 

 millimeters in width ; one of the two segments of collodion had a height 

 of 60 millimeters when the apparatus was filled with gas, the other 

 57.5 millimeters. 



The experiment was made by placing a watch at a certain distance 

 from the lens in the direction of its axis, and observing on the other 

 side the place where the tick was heard with the greatest intensity. 

 It was easy to perceive the tick very distinctly, when the ear was 

 placed in the axis, though it was scarcely perceptible when it was 

 removed. In another experiment the ear being located in a position 

 for hearing plainly, the lens was removed,, the ticking ceased entirely ; 

 upon replacing the lens it was again heard distinctly. 



To prevent illusion, the eyes of the observer were closed in such a 

 manner as not to perceive when the l^ns was removed or replaced. In 

 fine, it was observed that the point where the ticking ceased to be per- 

 ceptable in the direction of the axis of the lens became more and more 

 distant as the watch was brought nearer, though it was impossible to 

 assign with precision the point where the intensity was a maximum, 

 and consequently to verify the formulae relative to the foci of lenses. 



These experiments were not only made by M. Sondhauss, but by 

 other persons accustomed to observe ; MM. Bunsen, Kuchof, and 

 others, who obtained the same results. Besides, to remove all doubts, 

 M. Sondhauss sought to verify these results, without the aid of the ear, 

 which he accomplished in the following manner :-- Instead of the 

 watch the sonorous body employed, was the mouth piece of a flute or 

 flageolet, and the vibrations of the air through the len were observed 

 by means of a membrane stretched at the end of a kind of acoustic 

 cornet. The undulations arriving at the opening, were thrown upon 

 the membrane and were rendered apparent by the movement of a small 

 quantity of fine sand sprinkled upon its surface. In this way the 

 phenomena were observed very distinctly. 



He was able, in short, by means of the lens of carbonic acid, to 

 make an experiment analagous to that of the conjugate mirrors. Two 

 persons placed on opposite sides of the lens, were enabled to hold a 



