flame was substituted for that of daylight and its brightness 
varied by the method of Vierordt, i. e. by changing the width 
of the slit. At the low intensities thus obtained the fluctua- 
tions in the sensitiveness of the eye were so great as to defeat 
the object in view. The observations only served to confirm 
the fact already established, that with diminishing brightness 
the 
retinal image was measured, was, according to Talbot's law for _ 
the luminosity of rotating black and white disks, only +$z 4 
great as that of the spectrum of uninterrupted daylight, seen 
through the same instrument. The difference is about that 
between the intensity of light reflected by white paper, 0D the 
one hand, and from a lamp-blacked surface on the other. 
Since the relative sensitiveness of the eye for different wave 
lengths varies with the intensity of the ray, being proportion: 
ately much greater for the more refrangible rays when the 
intensity is small, the luminosity curve of a faint spectrum — 
* The relative luminosity of different portions of the spectrum as determined 
by Fraunhofer and Vierordt are given in the following table. Their results are 
the duration of the corresponding impression upon the retina, and beca os 
show that the luminosity differs for different eyes in much the same way 4% 
duration of the image is found to do. 
Luminosity of the Spectrum. 
Fraunhofer.! Vierordt.? 
5 Rae aie aa 32 Wie Ooo cid 
er ()) 94 ‘ Orange (C) ,------- 12 
ellow (D)_-...... 640 Yellow (D) -.------ 80 
Yellow (D to F)....1000 Yellow (D to E)----1000 
Green ioe ee a ee een {E) ......44- 370 
Blue-green (F) .--. 170 Blue-green (F) .---- 128 
lee (GF 5 side. Blue (G) 2... -b.0 <2 3 
Violet (89) 052. eee Violet (H) .------- 0° 
1 Denkschrift der Bayerischen Akademie, 1815. 
* Anwendung der Spectral-Analyse, Tibingen, 1871. 
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