36 H. Rubens and B. W. Snow on the Refraction of 



of the Fraunhofer lines the wave-lengths of the interference- 

 bands were determined for the visible portion of the spectrum, 

 and from these data were calculated the order m of each dark 

 band and the product K = 2dcosi. The knowledge of these 

 two constants proved, then, sufficient to determine also the 

 wave-lengths of the interference-bands in the infra-red. 



The positions of these latter we^e obtained by allowing the 

 sensitive filament of a linear bolometer to wander through the 

 spectrum, and plotting the observed galvanometer-deflexions 

 as a function of the angular deviation. The interference- 

 bands are then recognized as minima or maxima in the curve. 

 In this way, for a series of angular deviations may be deter- 

 mined the corresponding indices of refraction ; that is, a 

 number of points in the n — X plane determined, which, when 

 joined by a smooth curve, give the curve of dispersion for the 

 material examined. 



In the majority of the bodies thus investigated, the limit of 

 the region in the infra-red capable of being explored was 

 prescribed by the absorption, which increases rapidly with 

 increasing; wave-lengths. In two cases alone, viz. when 

 working with rock-salt and with fluorite, were the investiga- 

 tions discontinued at wave-lengths X=5'7 /jl and X = o'3/m 

 respectively, before the region of strong absorption was 

 reached. This was rendered necessary by the fact that the 

 apparatus employed proved to be insufficiently sensitive to 

 measure the exceedingly feeble energy found in the spectrum 

 of the zirconia burner at these long wave-lengths. 



As a means of continuing the investigation beyond this 

 point two ways of improvement suggested themselves. At 

 first we thought it possible to increase the energy of the source 

 of light, but all endeavours to attain this end proved of no 

 avail. The use of the electric arc for this purpose was, after 

 a short but thorough trial, discontinued. Even arc lamps of 

 unusually good regulation, when supplied by the almost per- 

 fectly constant current of the Berlin Central Station, gave a 

 radiation too fluctuating to be used in place of the zirconia 

 light. The regulation of the arc by hand was also tried, but 

 also without success. The use, moreover, of a zirconia burner 

 of nearly double the dimensions of the former resulted in only 

 a feeble increase in the energy, while a series of new diffi- 

 culties was thereby introduced, such as the melting of the 

 platinum cell, a greater consumption of gas, &c. We con- 

 cluded, therefore, for the further investigation, to retain the 

 source of light in its original form, and to make better use of 

 the energy here at hand by increasing as far as possible the 

 sensitiveness of the measuring apparatus. 



