Water and Mercury at nearly Perpendicular Incidence. 313 



will a fine adjustment by screening one or the other light 

 from the plate for a measured interval of time. In practice 

 the direct light was thus partially cut off, a mechanically held 

 screen being advanced a little above the plane of the revolving 

 disk. The reader will not fail to observe that the incomplete 

 coincidence of the times of exposure has the disadvantage of 

 rendering the calculation dependent upon the assumption that 

 the light is uniform over the duration of an experiment. 

 Error that might otherwise enter is, however, in great degree 

 obviated by the precaution of choosing the middle of the 

 total period of exposure as the time for screening. 



The above is a sufficient explanation of the general scheme, 

 but there are many points of importance still to be described. 

 With respect to the source of light, it was at first supposed 

 that even if the radiation upwards and downwards could not be 

 assumed to be equal, at any rate a reversal by rotation of the 

 lamp through 180° in the plane of the diagram would suffice 

 to eliminate error. On examination, however, it appeared 

 that owing to veins in the glass bulb the radiation in various 

 directions was very irregular, so much so that it was feared 

 that mere reversal might prove an insufficient precaution. 

 The difficulty thus arising was met by covering the bulb, 

 or at least an equatorial belt of sufficient width, with thin 

 tissue-paper, by which anything like sudden variations of 

 radiation with direction would be prevented, and by causing 

 the lamp to revolve slowly about its axis during the whole 

 time of exposure. The diameter of the bulb was about 1\ 

 inch, and the illuminating-power rather less than that of one 

 candle. 



Another point of great importance is to secure that the 

 light regularly reflected from the upper surface of the liquid, 

 which we wish to measure, shall be free from admixture. It must 

 be remembered that by far the greater part of the light inci- 

 dent upon the liquid penetrates into the interior, and must be 

 annulled or at any rate diverted into a harmless direction. 

 To this end it is necessary that the liquid be free from .tur- 

 bidity and that proper provision be made for the disposal of 

 the light after its passage. It is not sufficient merely to 

 blacken the bottom of the dish in which the water is con- 

 tained. But the desired object is attained by the insertion 

 into the water of a piece of opaque glass, held at such a 

 slight inclination to the horizon that the light from the 

 lamp regularly reflected at its upper surface is thrown to one 

 side. As additional precautions the disk and its mountings 

 were blackened, as were also the walls and ceiling of the room 

 in which the experiments were made. 



