On the Use of Gutta Fercha in Eleclrkcd Insulation. 165 



universal use. The objectof that method is to remove the indi- 

 stinctness produced by diffraction, by converging the illumina- 

 ting rays upon the object, so that they may again radiate from 

 the object as if it were self-luminous. This effect can only be 

 obtained by furnishing the microscope with an illuminating 

 apparatus as perfect as its magnifying apparatus. In Dr. 

 Wollaston's method there is no convergency of the rays upon 

 the object. Theraysdo not diverge fromthemargin of theaper- 

 ture, but from the source of light; and even if Dr.WoUaston 

 had directed that the illuminating rays should be converged 

 accurately to foci coincident with the points of the object to 

 be viewetl, a lens three-quarters of an inch in focal length was 

 quite useless for any such purpose. To try to make light 

 radiate from an object seen in a microscope with any reason- 

 able magnifying power by means of such a lens, would be as 

 absurd as to attempt to see the satellites of Saturn through an 

 opera-glass; but it would be still more absurd to make such 

 an attempt by an optical arrangement, under which the object 

 is illuminated by rays which diverge from a great or an infinite 

 distance, while the object itself is in the conjugate focus of a 

 perforation within two or three inches of tiie lens I 



Dr. Wollaston was too sagacious an observer, and had too 

 much knowledge of optics to make any such proposition. His 

 apparatus accomplishes perfectly the purposes which he con- 

 templated ; and it is no error of his, but only an error of his 

 commentators, that we have now been endeavouring to correct. 



St. Leonard's College, St. Andrews, 

 February 14, 1848. 



XXV. On the Use of Gutta Pcrcha in Electrical Insulation. 

 By Michael Faraday, F.R.S., Foreign Associate of the 

 Academy of Sciences, Sfc. 



Royal Institution, 



My dear Phillips, Feb. 9, 1848. 



I II AVE lately found gutta percha very useful in electrical 

 experiments; and therefore, that others may take advan- 

 tage of its properties if they have occasion or are so inclined, 

 give you this notice for insertion in the Philosophical Maga- 

 zine. Its use depends upon the high insulating power which 

 it possesses under ordinary conditions, and the manner in 

 which it keeps this power in states of the atmosphere which 

 make the surface of glass a good conductor. All gutta percha 

 is not however equally good as it comes from tlie manufac- 

 turer's hands; but it does not seem difiicult to bring it into 



