94 WheeU ■r — kxjwriiiK nhiJ Iiuvxii.yati.on on the Inflection 



nation than those used before; and second, in reducing the 

 possibility of a gaseous film between the mirror and the liquid 

 to a minimum. On account of the difficulties in the way of 

 certainly getting rid of films of surface contamination in solid 

 mirrors, it was decided to attempt to attain the first object by 

 using a mercury mirror. The advantages of such a mirror are, 

 first, it admits of obtaining an extraordinarily clean surface 

 much more easily than any other metal ; second, exactly simi- 

 lar surfaces can be easily and quickly reproduced ; and third, 

 the degree of cleanliness attained can be estimated better than 

 with mirrors of solid metal. The method of cleaning the sur- 

 face by stretching the dirt to the edges (first suggested, in ithe 

 case of water, by Lord Rayleigh)* was adopted in the form rec- 

 ommended by Rontgen.f This is described below. 



It was feared at first that the attainment of the second object 

 would present formidable if not insuperable difficulties. Drude 

 has suggested the possibility of removing the film of gas by 

 heating;;}: but this would involve obvious complications and diffi- 

 culties. As mentioned in the introduction, it seems probable 

 to expect that the gas film is introduced by entrainment in the 

 act of immersion. Hence it was decided to produce the clean 

 mercury surface while the mirror was immersed. The method 

 of Rontgen for producing the clean surface lends itself very 

 readily to this purpose, and it would seem, from the results ob- 

 tained, that complete success in this important object has been 

 attained. The form which the apparatus for producing the 

 mirrors finally took is shown in the figure. 



A glass tube of about 4 cm diameter was drawn down to form 

 a funnel-shaped vessel (A), into the neck of which was sealed 

 a small tube (B), having its end drawn out to a capillary 

 point and projecting into the funnel. This tube, passing 

 through a cork at the bottom of the cylindrical iron vessel (C) 

 which surrounded the funnel, was supported so that the mouth 

 of the funnel (ground to a plane) came slightly above the top 

 of the cylinder. Another tube (D) through the cork (which 

 was provided with a suitable stopcock, not shown in the figure) 

 carried away the mercury which overflowed the funnel. The 

 iron cylinder passed through the bottom of an open rectangular 

 iron box (E) and projected to within about l^ " 1 of its top. 

 This box served to hold the liquids under which the reflection 

 was observed. 



This surface-cleaning apparatus was mounted on a stand 

 furnished with levelling screws to enable one to adjust the 

 mouth of the funnel accurately to a horizontal plane. The 



* Phil. Mag. (5), xxx, p. 392, 1890. 

 + Wied. Ann., xlvi, p. 152, 1892. 

 % Wied. Ann., xxxix, p. 545, 1891. 



