176 KEPORT — 1892. 



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 eS'ected, 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 visibility should be as great 

 as possible, and was accordingly marked 100. The screw (of 1 mm. 

 pitch) was then turned through one turn, thus giving a difference of 

 path of 2 mm., and the visibility again estimated, and so on. The curve 

 was then drawn, giving the estimated visibihty for each 2 mm. difference 

 of path, and this was corrected for the personal equation as before 

 described. 



Hydrogen.^ 



The full curve in 'fig. Bh, Plate I., represents such a curve for the 

 red hvdrogen line at a pressure of about 1 mm. and a temperature of 

 about 50° C. 



The dotted curve represents 



V=2~^°'"' cos -7/30.2 



It follows that the visibility curve is practically the same as that due 

 to a double source, whose components have the intensity ratio 7 : 10, and 

 in each of which the light is distributed according to the exponential law, 

 expressed by the first term. 



The formula for a double soui'ce, where the components are similar, is 



l + ,.2 + 2rcos27r5 

 V 2= P V-, 



l + ,.2 + 2r 



in which D, the period of the curve, is inversely proportional to the 

 distance between the components. 

 But D = ]SIAi = (lSr + l)A2, whence 



Hence, in the present instance, we have for the distance between the 

 components of the red hydrogen line 



gV X (6-56 X 10~ V =1'4 X 10" Vm., 



or 0'14 division of Rowland's scale. 



Again, if 8 be the ' half- width ' of the spectral line (the value of x 

 when (f>(x)=^^), then 



</)(.-«) =2 r, and V=e — r.' 



' 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. 



- As frequent use is to be made of the function 



l+»-- + 2rcos2ir-'^ 

 D 



it will be abbreviated to the form cos r/D. 



