E. 8. Ferry — Persistence of Vision. 



199 



lire of luminosity is the amount of light necessary to enable one 

 to clearly distinguish objects. The spectra of various light 

 sources are so very different in the distribution of luminosity, 

 that it was considered necessary to determine these values for 

 the particular lamp used in these experiments. 







































































































































\° 









? 





























i 

















































































\ 















Wave 



Lenatl 













v 





3 30 350 37 



390 6(0 630 650 670 600 



Distribution of Luminosity in 16-candle power, 100-volt Edison Incandescent Lamp 



The method employed was to insert an object into the eye 

 piece of the spectrometer and to reduce the aperture of the 

 objective of the observing telescope by means of a tine microm- 

 eter slit till the object was just visible. The reciprocals of 

 the micrometer slit areas gave the relative luminosities of the 

 different parts of the spectrum. This method is less conve- 

 nient and possibly less accurate than the Eumford photometer 

 method used by Abney and Festing* but it gave a probable 

 error of only about five per cent. 



Table III. 

 Distribution of Luminosity in normal spectrum of a 16 C. P. 100 volt Edison 

 Incandescent lamp — Platted in Fig. 2. 



Wave- Relative Wave- Relative 



length. luminosity. length. luminosity. 



•435 1-86 -589 100'UO 



•455 3-06 -615 83*25 



•480 13-89 -645 54-37 



•510 28-28 -675 17*12 



•540 50-00 -684 11 "16 



•570 89-25 



* Colour Photometry, Trans. Roy. Soc. Lond., 1888, p. 547. 



