566 CANDLE-POWER OF ARC LAMPS [Cn. XIII 



90 Right-angle arc (dotted lines). 



55 Arc with V-arranged carbons (full lines). 



The numerals around the semicircle represent degrees, while those along the 

 middle radius represent candle-power. It is to be noted that with the V- 

 arrangement where both craters supply light that there is considerable gain 

 over the right-angle arrangement. 



image becomes too large to enter the opening (objective front), 

 there is no advantage to be gained by increasing the current, as 

 this merely increases the size and not the brightness of the crater 

 and the crater image. 



774. Visible and invisible radiation. It is a well known fact 

 that, of the total energy supplied to an arc lamp, but a small part 



FIG. 307. NORMAL SPECTRUM ILLUSTRATING THE SEGMENT OF RADIATION 



WHICH is VISIBLE. 



The longest radiation represented in this diagram has a wave-length of 2 f-< 

 and is at the base of the triangle. The intermediate wave-lengths occur in 

 regular sequence. 



The segment of visible radiation occurs between wave-lengths .68 M and .40 M. 

 Other waves shorter than .40 n form the ultra-violet, and those longer than 

 .68 M the infra-red part of the spectrum. 



Under some conditions waves longer than .68 M and shorter than .40 M may 

 be seen, but the radiation for useful vision falls between those wave-lengths. 

 The height of the lines in this diagram represents the wave-lengths magnified 

 20,000 times at that particular point in the spectrum. 



If the visible radiation is passed through a prism or a diffraction grating, the 

 wave-lengths are arranged in regular sequence from the longest to the shortest 

 as shown in the diagram. The longest visible waves appear red to the normal 

 eye and the shortest violet, with the orange, yellow, green, blue, and indigo in 

 between. 



773a. Blondel, Proceedings of the International Electrical Congress. 

 Chicago, 1893. 



Bulletin of the Bureau of Standards, Vol. r, p. 122 and reprint 8. 



