Regular Reflexion of Light by an Absorbing Gas. 335 



us that the reflecting power of a surface of dense mercury 

 vapour for the light of the 2536 line is not far from 

 25 per cent., nearly that of most metals in the same region 

 of the spectrum. The next point to determine was the 

 density at which selective or metallic reflexion commenced. 

 Various methods were tried, the following being the one 

 finally adopted. 



It is of course desirable to repress as completely as possible 

 the ordinary, or vitreous, reflexion of the quartz surface. 

 This can be done by polarizing the incident beam with its- 

 electric vector horizontal, and setting the prismatic face of 

 the bulb at the polarizing angle. Under these conditions the 

 intensity of the two beams reflected from the outer and the 

 inner surface is reduced nearly to zero, and a very slight 

 increment in the reflecting power of the inner surface due 

 to the mercury vapour becomes at once apparent provided 

 the reflecting power is increased regardless of the direction 

 of the plane of polarization, as proved to be the case. 



The divergence of the two reflected beams was so great 

 that both were not received by the quartz lens of the camera, 

 consequently the one reflected from the outer surface was 

 made nearly parallel to the other by reflexion from a piece 

 of platinized glass at nearly grazing incidence. Two small 

 images of equal intensity were thus recorded simultaneously 

 on the photographic plate, one above the other. The upper,, 

 due to reflexion from the outer surface of the prismatic plate; 

 the lower, representing the reflexion from the inner surface. 

 The temperature of the bulb was now gradually raised, and 

 a number of exposures of equal duration made at different 

 temperatures, the plate being moved slightly between the 

 exposures. Fig. 5, Plate VI. shows the result of the final 

 experiment. The first three exposures, "a," " b" and u c/ ; cor- 

 responding to temperatures of 180°, 210°, and 235°, showed 

 both images of equal intensity; in other words, no increment 

 of reflecting power resulted from the presence of mercury 

 vapour up to a pressure of 50 mm. (at 235°). Exposure "d" 

 taken at a temperature of 279°, showed the lower image con- 

 siderably brighter than the upper one. The pressure in this 

 case was about 120 mm.; and since we can infer that the 

 effect would be noticeable at a slightly lower pressure, we 

 are safe in saying that the first appearance of specular 

 reflexion by the vapour takes place at a pressure not very 

 far from 10 cm. Exposures "e" and "/" were made at tempe- 

 ratures of 300° (pressure 25 cm.), and one other still higher 

 temperature which was beyond the reach of the thermometer. 

 On the print these have practically the same intensity, but 



