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CAPTAIN W. BE W. ABNEY AND MAJOR-GENERAL E. R. TESTING 
mined. When the last was used the intensity of c was almost exactly y-gro of that of 
a. As the rotating sectors gave a further extreme reduction of, say, yj, a could be 
used of a manageable intensity. 
Fig. 40. 
Method of double reflection into extinction box. 
When employing this method the collecting lens in front of the spectrum was so 
adjusted, that the re-combined beam from the whole spectrum formed a circular spot 
on S, the position of the spot of light on S was, therefore, the same for all parts of 
the spectrum. 
The absolute luminosity of the beam from D of the spectrum was measured by 
placing an open screen at the same distance from the mirror M (fig. 38) that S was, 
two silvered mirrors being used at A and B, and using the amyl-acetate lamp for 
comparison. The absolute luminosities of beams from other parts of the spectrum 
were then calculated from this by means of the luminosity curves, fig. 33. 
The results obtained by using the rotating sectors with this apparatus were also 
tested by the method before described, and were found to be perfectly trustworthy. 
From the observations made, a curve was plotted showing what was the proportion 
of the beam from each part of the spectrum which was just not visible. The absolute 
luminosity of each part of the spectrum having been determined in the way explained 
above, a second curve was plotted of which the ordinates represent the absolute 
luminosity of each part of the spectrum at the extinction point, or, in other words, 
the proportion which would be just not visible, supposing that eacli part had been 
originally of the uniform luminosity of, say, one candle. This curve rose from the 
blue-green towards the red, when, after reaching a maximum, it tended to drop again. 
There appeared to be a similar irregularity at the violet end. It was suspected that 
these irregularities might be caused by some admixture of white light due to want of 
perfect transparency of the prisms, and further investigation showed that this was 
the case, and that when this stray white light was eliminated the curve became of 
the form shown by the dotted line, fig. 41. 
Helmholtz’s plan of dispersing this white light was first tried. A prism was placed 
in the path of the beam from the collecting lens at such a distance that the beam 
