﻿702 Prof. R. W. Wood on Selective Reflexion, Scattering 



presence of the air. The intensity of the secondary reso- 

 nance radiation in comparison with that of the primary will 

 consequently be much less (one-third) than when the mercury 

 vapour was in a high vacuum. This hypothesis was tested 

 by experiment and practically proven. In the first place a 

 very careful series of measurements was made of the re- 

 duction in the inteusity of the primary resonance radiation 

 by the introduction of air. The pressure of the air was 

 measured with a McLeod gauge, and the duration and 

 intensities of the excitation were made as nearly equal as 

 possible. 



The exposures were all made on the same plate, which was 

 pushed along in the supporting clamps, the mercury himp 

 being allowed to cool down completely between exposures. 

 To insure against accidental errors, a large number of plates 

 were exposed, and the measurements made from each were 

 compared. Oue of these plates is reproduced on PL XI. fig. 12. 

 The air pressure in the cell is marked on each picture. The 

 exciting beam enters the cell from the right, and two thirds 

 of the quartz window was screened off, so that a number of 

 exposures could be made on the same plate. In the first 

 picture (pressure O'OL mm.) the secondary radiation from 

 the region not excited by the primary beam is very con- 

 spicuous, less so iu the second and nearly gone in the third. 

 The intensities of the primary radiation at the point where 

 the incident beam entered the cell w T as measured by com- 

 paring the density of the negative with the density of a 

 plate exposed in strips for times increasing gradually from 

 5 to 300, which plate was developed simultaneously with the 

 other. The comparison was made in a very simple way 

 which, with practice, was susceptible of considerable accuracy. 

 A clean cut was made with a sharp knife through the film 

 on each plate, perpendicular to the direction of the strips on 

 the comparison plate, and through the region at which the 

 density was to be measured on the other plate. The film 

 was removed along one side of the cut in each case, exposing 

 the clear glass. The two plates were now placed in contact 

 film to film, and the comparison strip slid along until a 

 perfect density match was made with the photograph, the 

 dividing line between the two disappearing. In practice it 

 was found advantageous to cover the plates with a black 

 card perforated with a small rectangular aperture, which 

 was brought over the region under investigation, and the 

 best match could be made by throwing the eye a trifle out 

 of focus, thus causing the very narrow line separating the 

 two halves of the field to disappear. 



