Interference Methods to Spectroscopic Measurements. 287 



the secondary of a large induction-coil, whose primary cur- 

 rent was interrupted by a rotary break attached to the arma- 

 ture of an electric motor, making about 20 to 30 breaks 

 per second. The steadiness of the light thus obtained was far 

 greater than with the ordinary Foucault interrupter. Probably 

 it would have been still more satisfactory to use an alternating 

 dynamo properly wound to give a strong current with compa- 

 ratively few alternations. 



The box surrounding the vacuum-tube was heated just suffi- 

 ciently to give a steady bright light, and the temperature then 

 kept as nearly uniform as possible. This temperature was 

 usually taken to represent that of the vapour within the tube. 

 This is, of course, only a rough approximation to the truth ; 

 and in some cases the estimate was much too low. 



As it was not intended to include in the present work an 

 elaborate study of the effect of temperature, this matter was 

 not of great consequence. It may be suggested, however, that 

 a very much closer approximation to the real temperature could 

 be obtained by winding a platinum wire about the capillary 

 portion of the tube, and deducing the temperature from the 

 variation of its resistance. A preliminary experiment in which 

 a platinum wire passing through the tube and heated by a 

 current until the platinum spiral outside the tube was raised 

 to fixed temperatures, would give a means of deducing, from 

 the indications of the spiral, the true temperature within the 

 tube. 



These adjustments being effected, the screw of the " wave- 

 comparer " was turned to zero ; that is, till there was no dif- 

 ference of path between the interfering pencils. At this 

 point the visibility should be as great as possible, and was 

 accordingly marked 100. The screw (of 1 millim. pitch) was 

 then turned through one turn, thus giving a difference of path 

 of 2 millim., and the visibility again estimated, and so on. 

 The curve was then drawn, giving the estimated visibility for 

 each 2 millim. difference of path ; and this was corrected for 

 the personal equation, as before described. 



Hydrogen. 



The full curve in fig. 3 b, PI. V., represents such a curve 

 for the red hydrogen-line * at a pressure of about 1 millim. 

 and a temperature of about 50° C. 



* The hydrogen was prepared by dropping distilled water upon sodium 

 amalgam, and allowing the gas to pass through sulphuric acid into the 

 vacuum-tube, which was repeatedly exhausted until the spectrum of 

 hydrogen was nearly pure. 



X2 



