242 



SCIENCE. 



the recent book " How to see with the Microscope," 

 a work which is a valuable addition to Microscopical 

 Literature ; both wrote articles against " The Mis- 

 interpretation of the Microscope?' In that article 

 we gave very strong illustrations of the " misrepresen- 

 tation?, " referred to, but the paper was written some 

 years in advance of the present developments, which 

 have made the case much stronger. The disputed 

 resolution of the " Podura " scale was then quoted as 

 an instance of an objective giving two distinct resolu- 

 tions of an object, one of which was clearly an errone- 

 ous one, but who would have then anticipated that 

 the spherules on " Angulatum" which we have for so 

 many years religiously regarded as the true ultimate 

 resolution of that diatom, would prove to be an illu- 

 sion ? While to make the case more complicated, Pro- 

 fessor E. Abbe states that " while it is not my opinion 

 that the Angulatum valve is composed of spherules, 

 yet even if such should exist, they would not have a 

 different effect." 



Thus " The Misinterpretation of the Microscope " 

 under certain conditions, is no myth, but an admitted 

 fact • we welcome then the .improvements which shall 

 at least partially remedy the evil. The high angle 

 objectives of the present, although far from perfect, 

 give great hope for the future, and we trace in 

 Professor Smith's work, to which reference has been 

 made, the advent of a higher intelligence among 

 Microscopical workers. This new spirit of progress 

 is well described by Dr. Beale when he says, the 

 Microscopist, like the Astronomer, is ever longing 

 to get a little beyond the point at which he has 

 already arrived. Each new fact gained by research 

 seems but to indicate the existence of more and 

 more important things beyond. Limit is reached 

 and then surmounted, but soon a new limit seems to 

 rise from the mists in the distance towards which the 

 worker is impelled by new hopes and desires. It is 

 this never-halting progress which distinguishes scien- 

 tific from every other kind of inquiry, and particularly 

 microscopical investigation, for it can never be com- 

 pleted. It deals with the illimitable. The boundaries 

 of to-day are found to have vanished to-morrow, and 

 the eyes and understanding begin to penetrate into 

 regions which but a short time before had been con- 

 sidered far beyond the range of possible investigation. 



CoNDUCTIBILITY OK GLASS FOR THE GALVANIC CURRENT. 



According to A. Sewarz, if two platinum wires are inter- 

 posed in the same circuit, the one passing through the free 

 air while the other lies between two glass plates, or is 

 melted into a thick capillary tube, at a certain temperature 

 of the tube the former glows brilliantly, while the second 

 remains dark. If the glass becomes heated the former 

 grows dark, whence the author concludes that the glass has 

 become more conductive. 



THE PRODUCTION OF SOUND BY RADIANT 



ENERGY* 



By Alexander Graham Bell. 



In a paper read before the American Association for 

 the Advancement of Science, last August, I described 

 certain experiments made by Mr. Sumner Tainter and 

 myself which had resulted in the construction of a " Pho- 

 tophone," or apparatus for the production of sound by 

 light ; t and it will be my object to-day to describe the 

 progress we have made in the investigation of photo- 

 phonic phenomena since the date of this communica- 

 tion. 



In my Boston paper the discovery was announced that 

 thin disks of very many different substances emitted 

 sounds when exposed to the action of a rapidly-inter- 

 rupted beam of sunlight. The great variety of material 

 used in these experiments led me to believe that sonor- 

 ousness under such circumstances would be found to be 

 a general property of all matter. 



At that time we had failed to obtain audible effects from 

 masses of the various substances which became sonorous 

 in the condition of thin diaphragms, but this failure was 

 explained upon the supposition that the molecular 

 disturbance produced by the light was chiefly a surface 

 action, and that under the circumstances of the experi- 

 ments the vibration had to be transmitted through the 

 mass of the substance in order to affect the ear. It was 

 therefore supposed that, if we could lead to the ear air 

 that was directly in contact with the illuminated surface, 

 louder sounds might be obtained, and solid masses be 

 found to be as sonorous as thin diaphragms. The first 

 experiments made to verify this hypothesis pointed to- 

 wards success. A beam of sunlight was focussed into 

 one end of an open tube, the ear being placed at the 

 other end. Upon interrupting the beam, a clear, musical 

 tone was heard, the pitch of which depended upon the 

 frequency of the interruption of the light and loudness 

 upon the material composing the tube. 



At this stage our experiments were interrupted, as cir- 

 cumstances called me to Europe. 



While in Paris a new form of the experiment occurred 

 to my mind, which would not only enable us to investi- 

 gate the sounds produced by masses, but would also per- 

 mit us to test the more general proposition that sonorous- 

 ness, tender the znfluence of intermittent light, is a prop- 

 erty common to all matter. 



The substance to be tested was to be placed in the in- 

 terior of a transparent vessel, made of some material 

 which (like glass) is transparent to light, but practically 

 opaque to sound. 



Under such circumstances the light could get in, but 

 the sound produced by the vibration of the substance 

 could not get out. The audible effects could be studied 

 by placing the ear in communication with the interior of 

 the vessel by means of a heating tube. 



Some preliminary experiments were made in Paris to 

 test this idea, and the iesults were so promising that they 

 were communicated to the French Academy on the nth 

 of October, 1880, in the note read for me by Mr. Antoine 

 Breguet.J; Shortly afterwards I wrote to Mr. Tainter, 

 suggesting that he should carry on the investigation in 

 America, as circumstances prevented me from doing so 

 myself in Europe. As these experiments seem to have 

 formed the common starting point for a series of inde- 

 pendent researches of the most important character, 

 carried on simultaneously, in America by Mr. Tainter, 



*A Paper read before the National Academy of Arts and Sciences, 

 April 21, 1881. 



t Proceedings of American Association for the Advancement of Science, 

 Aug. 27, 1880 ; see, also, American Journal of Science, vol. xx, p. 305; 

 Journal of the American Electrical Society, vol. iii, p. 3; Journal of the 

 Society of Telegraph Engineers and Electricians, vol. ix, p. 404 ; Annales 

 de Chimie et de Physique, vol. xxi. 



% Comptcs Rendus, vol. xcl, p. 595. 



