462 W. JR. Grove, Esq, on [Jan. 29, 



the glass. Some of the plates had been kindly lent by him for this 

 evening ; and in one, Mr. Grove had removed a portion of the 

 backing, and the continuation of the gilded design came beautifully 

 out by breathing on the glass while in the frame of the electric 

 lamp, and was projected (as were the previous electrical images) 

 on a white screen. The effect on Mr. Babbage's plates may be also 

 electrical, arising from the gold— a good conductor — acting as pla- 

 tinum does in the voltaic battery, and setting up a chemical action 

 between the substance used for making the gold adhere and the 

 glass, or between the constituents of the glass itself ; but it would 

 be hazardous, without further experiment, to express any confident 

 opinion on this point. 



Of the practical results to science of the molecular changes 

 forming the subject of this evening's discourse, a beautiful illustration 

 was afforded by the photographs of the moon by Mr. Warren De la 

 Rue, which gave, by the aid of the electric lamp, images of the moon 

 of six feet diameter, in which the details of the moon's surface were 

 well defined, — the cone in Tycho, the double cone in Copernicus, 

 and even the ridge of Aristarchus, could be detected. The bright 

 lines, radiating from the mountains, were clear and distinct. A 

 photograph of the planet Jupiter was also shown, in which the belts 

 were very well marked, and the satellites visible. The following 

 question was suggested by Mr. Grove. As telescopic power is 

 known to be limited by the area of the speculum or object glass, 

 even assuming perfect definition, as the light decreases inversely as 

 the square of the magnifying power, a limit must be reached at 

 which the minute details of an object become lost for want of light. 

 Now, assuming a high degree of perfection in astronomical pho- 

 tographs, these may be illuminated to an indefinite degree of 

 brilliancy by adventitious light. With a given telescope, could a 

 better effect be obtained by illuminating the photographic image, 

 and applying microscopic power to that, than by magnifying the 

 luminous image in the usual way by the eye-glass of the telescope ? 

 Can the addition of extraneous light to the photograph permit a 

 higher magnifying power to be used with effect than that which 

 can be used to look at the image which makes the photographic 

 impression ? In other words, is the photographic eye more sensitive 

 than the living eye ; or can a photographic recipient be found which 

 will register impressions which the living eye does not detect, but 

 which, by increased light or by developing agents, may be rendered 

 visible to the living eye ? Much may be said, pro and con, on this 

 question, and it probably can only be satisfactorily answered by 

 experiment, when photographic science is sufficiently advanced. 



The phenomena treated of this evening, which are a mere selec- 

 tion from a crowd of analogous effects, show that light and electri- 

 city, in numerous cases, produce a molecular change in ponderable 

 matter affected by them. The modifications of the supposed im- 

 ponderables themselves have long been the subjects of investigation ; 



