52 



Prof. S. P. Langley on the 



would be desirable. As they have never been printed, 

 however, and since, so far as we know, none other such exist, 

 we will give them here under the cantion that they are to be 

 considered only first approximations. The light was that 

 from the pit of the positive carbon of gas coke, one inch in 

 diameter, with the current derived from a dynamo, actuated 

 by an engine of ten horse-power, and therefore certainly at 

 least as intrinsically hot and bright as any smaller arc-light 

 in more common use, and presumably much more so. The 

 apparatus was that already described in the memoir " On 

 hitherto unrecognized Wave-lengths." * 



The following Table gives the observed galvanometer de- 

 flexions after applying a multiplying factor for the shunt, 

 which had to be used for the larger readings : — 



1. 



2. 



3. 



4. 



5 



6. 





Deviation 

 Eock Salt 

 60° Prism. 



Wave- 

 length. 



Observed 



deflexion 



sun after 



absorption. 



Observed 



deflexion 



arc. 



Calculated 



deflexion 



sun without 



absorption. 



Observed 

 ratio sun 

 and arc. 



Calculated 



ratio of 

 unabsorbed 

 sun and arc. 





A* 



div. 



div. 



div. 







43 53 



0-373 



7-5 



1-3 



34 



5-77 



260 



43 17 



0-398 



11-5 



1-5 



38 



767 



21-8 



41 54 



0-489 



33-5 



5-3 



60 



6-32 



144 



41 05 



587 



1040 



10-5 



115 



9-90 



111 



40 45 



0663 



2040 



22-0 



204 



927 



10-0 



40 27 



0-749 



4320 



435 



450 



9-93 



91 



40 05 



0-96 



1 073-0 



215-0 



1 763 



4-99 



8-2 



39 54 



113 



1 783-0 



459-0 



3 534 



3-88 



7-7 



39 20 



2-87 



1 9050 



882-0 



5 645 



2-16 



6-4 



39 00 



43 



521-0 



235-0 



1 363 



2-22 



5-8 



38 45 



5-6 



75-0 



157-0 



832 



0-48 



5-3 



38 00 



104 



195 



39-0 



156 



0-50 



4-0 



The result of the comparison of the (of course unabsorbed) 

 electric arc with the radiation of the sun after absorption, as 

 shown in the sixth column, is that this solar radiation in the 

 orange and red is nearly ten times that of the arc, while to- 

 wards the violet end of the spectrum the relative superiority 

 of the absorbed solar heat diminishes, evidently because of the 

 progressive increase of the atmospheric absorption in tbat 

 direction, which lessens the solar intensity without sensibly 

 affecting that of the arc. The solar efficiency continues greater 

 through all the infra-red spectrum known until very lately, 

 while in the extreme portions, recently investigated, it falls 

 below that of the arc. This is partly due to the fact that radia- 



* See this Journal, xxii. August 1886. 



