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XLIV. Historical JVote on the Discovery of the 

 Ultra-microscopic Method . 



To the Editors of the Philosophical Magazine. 

 Gentlemen, — 



IT is essential that, when the work of earlier investigators 

 is found to have anticipated the " discoveries " of later 

 workers, due justice be done to the former. That is the 

 reason why I venture to draw the attention of the readers of 

 the Philosophical Magazine to the forgotten pages of the 

 ' Indian Engineering/ a weekly journal, in the numbers for 

 April 7th, 14th, and 21st, 1888*, of which were published 

 three papers by Mr. G. Dubern. In these papers was de- 

 scribed a method devised by him for observing and studying 

 the properties of particles too small to be visible under 

 microscopes even of the highest powers, when used with the 

 ordinary arrangements for illumination. The method briefly 

 described was the following :— The particles were held on a 

 glass plate under a microscopo ; a powerful beam of con- 

 vergent light entered the glass plate obliquely through a 

 polished slant-end-surface, and being incident at an ano-lo 

 greater than the critical angle on the surface of the plate, 

 suffered a series of total internal reflexions inside the glass 

 plate. Under these circumstances, the ultra-microscopic 

 particles appeared as tiny bright specks on a dark field. 

 Mr. G. Dubern's arrangement is identical in all particulars 

 with Mr. Cotton's modification of the ultra-microscopic 

 method of Siedentopf and Sxigmondy, and a comparison 

 between the figures illustrating Mr. Dubern's arrangement 

 in the ' Indian Engineering/ and fig. 306 on page 491 of 

 Prof . Wood's 'Optics/ representing Mr. Cotton's apparatus, 

 will convince even the sceptical that Mr. Dubern's apparatus 

 is fully entitled to the name " Ultra-microscope/'' and that 

 he must be regarded as having more or less completely 

 anticipated Siedentopf and Szigmondv. 



I have the honour to be, Gentlemen, 



Your most obedient Servant, 



a . A . ^ T , C. Y. Pamax. 



science Association Laboratory, 

 Calcutta, 17th December, 1908. 



