240 Prof. L. P. Wheeler on the Reflexion of 



through the cork (which was provided with a suitable stop- 

 cock, 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 1*5 cm. of its top. This box served 

 to hold the liquids under which the reflexion 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 

 stand was also provided with a rack and pinion to give the 

 vertical adjustment necessary to bring the reflecting surface to 

 the centre of the spectrometer table. A reservoir of mercury 

 hung at a higher level than that of the funnel was connected 

 with the tube (B) by a flexible tube provided with a suitable 

 pinchcock. In order to secure a mirror sufficiently free 

 from vibration, it was found necessary to mount the stand in 

 a Julius suspension. 



The operation of producing a clean surface was simply to 

 allow the mercury to overflow the edge of the funnel rapidly 

 for a few seconds. If care is taken to make the overflow 

 uniform all around the edge (accomplished by adjusting 

 the levelling-screws), surfaces of extreme cleanliness can 

 easily be obtained. The test of the cleanliness is the beha- 

 viour of a drop of water placed on the surface. In making 

 the test it is essential that the drop itself be clean. This is 

 most easily done in practice by dipping a piece of platinum 

 wire (cleaned by heating) beneath the surface of a beaker 

 of distilled water, and removing the adhering drop quickly 

 to the mercury surface. If the surface is really clean, the 

 drop is immediately stretched out to the edge in a film of 

 invisible thickness. The appearance of a mercury surface 

 to the eye is no test at all of its cleanliness ; it may seem to 

 be a perfect mirror and yet the drop remain in a hemi- 

 spherical form when placed on it, unless it has been recently 

 cleaned in the manner described. It is not necessary to 

 start with especially clean mercury— it cleans itself in a 

 very short time. Nor is it necessary to test the cleanliness 

 of the surface except after each fresh filling of the reservoir. 

 A good surface gets contaminated in the air in a brief time, 

 but retains its cleanliness long enough to permit the neces- 

 sary observations to be taken comfortably ; while a fresh 

 surface can be obtained easily and quickly. 



Thus it is reasonably certain that the reflecting surfaces 

 used were free from surface contamination. Of course it is 

 not so certain, a priori, that the film of gas was removed 

 when the mirror was submerged. It is natural to expect, 



