Comparison of Sources of Artificial Illumination. 107 



Special measurements of the " long-arc " lamp were made to 

 include this violet band. No break in the curve of intensities 

 was found until the edge of the band was reached, the relative 

 brightness at I =4330 being scarcely three times that of the 

 standard. From this point it rose at once to more than twenty 

 times that of the standard, a quantity approximating to that of 

 daylight in the same region. Beyond 1=4250 the values fell 

 again into the continuation of the original curve. The results 

 of these measurements are also given in Table II and curve III, 

 figure 2. 



Table II (the Electric Arc). 







Long-arc* (10-5 amperes, 52 volts). 





Short-arc 21 am- 

 peres 27 - 5 volts 











Above the horizon. 



Wave-lengths. 



a=30°. f 



Below the horizon. 



a=— 16°.f 





Electric arc 



Electric arc}: 



Electric arc 





Standard lamp. 



Standard lamp. 



Standard lamp. 



7530 



8-50 



036 



0-55 



6680 



0-71 



055 



0-66 



6080 



0-88 



0-90 





5890 



1-00 



1-00 



1-00 



5570 



1-26 



1-26 





5185 







1-63 



1-96 



4920 







1-73 



2-85 



4685 



2-08 



2-03 



3-88 



4500 



2-50 



2-51 



4-64 



4340 





2-80 







4250 



- - . - 



20-40 







4100 







3-22 







The results of measurements of the light from the " Fou- 

 cault regulator" arc-lamp, by A. Crova,§ W. H. Pickering | 

 and H. C. VogeL^f reduced to a common scale for purposes of 

 comparison, are incorporated in Table III. Crova's standard 

 of comparison was a moderator lamp burning colza oil, concern- 

 ing the color of the flame of which we only know that it was 

 slightly redder than that of the stearine candle. The set of 

 values given in Table III, are obtained from his measurements 

 without taking into consideration the quality of his standard 

 lamp. They are between those of Pickering and Yogel. 



None of these determinations take cognizance of the violet 

 band at X 4280. They are introduced here because they afford 

 a comparison of the results of three distinct methods of spectro- 



* Length of arc =3 - 5 mm . 



f a is the angle which the line, joining the block of magnesium carbonate with 

 the arc, makes with the horizontal plane. 



\ Mean of values for a=0°, a=14°, a=28° and <z = 50°. 

 § A. Crova, Comptes Rendus, t. lxxxvii, p. 322, 1878. 

 || W. H. Pickering, Photometric Researches, etc., p. 246. 

 ■[T H. C. Vogel, Berliner Monatsberichte, 1880 ; p. 801. 



