TO SPEOTKOSCOPIC MEASUKEMENTB. 11 



The vapor was rendered hxmiiioiis l>y the discharge from the secondary 

 y>t a large induction-coil, whose primary cui-rent was inten-upted by a rotary 

 break attached to the armature 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 comparatively few alternations. 



The box surroimding the vacuum-tube was heated just sufficiently to 

 give a steady, bright light, and the temperatiu'e then kept as nearly uniform 

 as possible. This temperature was usually taken to represent that of the 

 vapor 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 temperatm-e, this matter was not of great consequence. It 

 may be suggested, however, that a very much closer approximation to the 

 real temperatm-e could be obtained by winding a platinum wire about the 

 capillary portion of the tiibe, and deducing the temperatm-e fi'om the varia- 

 tion of its resistance. A preUminary experiment in which a platinum wii-e 

 passing through the tube and heated by a current until the platinum spiral 

 outside the tube was raised to fixed temperatm-es, 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 difference of path between the 

 interfering pencils. At this point the visibihty should be as great as pos- 

 sible, and was accordingly marked 100. The screw (of 1 '"'"• pitch) was then 

 turned through one tiim, thus giving a difference of path of 2 '"'"•, and the 

 visibihty again estimated, and so on. The curve was then drawn, giving 

 the estimated visibility for each 2 """■ difference of path, and this was cor- 

 rected for the personal equation, as before described. 



Hydrogen. The full cm-ve in Fig. 3b, Plate II., represents such a curve 

 for the red hydrogen Hue* at a pressure of about 1 ■"■"• and a temperature of 



about 50° C. The dotted cm*ve represents V=2~ "" cos 0.7 30. 



* The hydrogen was prepared by dropping distilled water upon sodium amalgam, and allowing the 

 gas to pass through sulphuric acid into the vacuum-tuVje, which was repeatedly exhausted until the 

 spectrum of hydrogen was nearly pure. 



