400 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



for the successive phases of the spark which he may wish to photograph ; 

 not only so, but he can repeat any phase as many times as he likes, and 



thus obtain a strong com- 

 posite in cases where a 

 single exposure would 

 produce no visible effect. 



In this manner we have 

 photographed the spark 

 spectrum of carbon, with 

 a ten-foot concave grating, 

 in nine different phases,* 

 which may be roughly de- 

 scribed as follows : — 



1. Exposure begins £ 

 second after breaking arc 

 and lasts J second. Here the carbon poles are still white hot and the 

 spark is practically silent when compared with the noise which the cold 

 spark makes. In this stage the luminosity is so exceedingly feeble that, 

 with a slit of the same width as in the rest of the series, six to ten hour? 

 (i. e. about 5000 exposures) are required to get a fair negative. 



Figure 3. 



Figure 4. 



Figure 5. 



2. Exposure begins ^ second after breaking of arc and lasts for 1 

 second. The middle of the exposure, therefore, occurs f second after 

 the beginuing of the spark. Here, again, the image of the spark on the 

 slit of the spectroscope is quite invisible during the entire exposure. 



* The purpose of this experiment, it will be observed, is therefore fundamen- 

 tally different from that in which Sir Norman Lockyer examined the spark spectra 

 of salts volatilized in flames and which he described in Proc. Roy. Soc, 30, 

 pp. 22-31 (1879). 



