360 CAPTAIN, W. T)E W. ABNEY AND DR. T. E. THORPE ON PHOTOMETRIC 
lamp was then wa.rmed from 55° to 75°; the flame became longer, but when turned 
down to the height of the gauge the same value was obtained against the glow lamp as 
before within 2 per cent. Another photograph was taken of the flame from the same 
position, and the two compared. The flames in both cases were equal in dimensions. 
In the paper last referred to it was also shown that either the Bunsen or the 
Rumford method of photometry could be adopted. The method of Rumford is 
undoubtedly better than that of Bunsen when the lights are very different in colour, 
as in the latter method there is a certain thickness of translucent material through 
which both lights have to pass, and only after such passage can equality of illumina¬ 
tion be estimated; and if the paper employed for the screen is coloured in any degree, 
this must of necessity affect the results. The light of the corona and that of the glow 
lamp are very different in colour, the former being stronger in the blue end of the 
spectrum than the latter. It must be recollected that the greatest luminosity is in 
the yellow of the spectrum in both cases, and, though the blue end of the spectrum 
alters the hue, it has very small effect on the luminosity. This being the case, it was 
thought that no error of any magnitude would be introduced by adopting the Bunsen 
plan, since the brightest part of the two spectra would be compared with one another. 
It was evidently impracticable to adopt the PlUMFORU method in the apparatus in 
which the intensity of different points in the corona had to be measured. For this 
purpose a telescope by Simms, lent by the Astronomer Royal, was employed. The 
object glass had a focal length of 78 inches and an aperture of 6 inches, thus forining 
an image of the Moon ’76 inch in diameter. The image was received on a circular 
white screen contained in a photometric box and placed exactly in the focus of the 
object glass. In the centre of the screen was traced a circle of the diameter of the 
image of the Moon, and during the observation the Moon’s disc wms made to fall 
exactly within the circle. As the telescope was equatorially mounted with clock¬ 
work, the image was kept stationary within the circle. The screen was of Rives’ 
paper of medium thickness, and round the pencil-circle a series of small grease spots 
about of an inch in diameter had been made. There was some difficulty in pre¬ 
paring these small grease spots, but a method was eventually devised which answered 
admirably. Faint pencil lines were drawn radially from the centre of the circle, and 
the places where each spot was to he produced were marked with a dot. White 
blotting paper was soaked in spermaceti, any excess being avoided. Small discs, 
■g- inch, were punched out, and these discs were put centrally on the dots. Blotting 
paper was next placed over them when in position, and a hot flat-iron was j^assed 
over them. The blotting paper and the small discs were then removed, and clean 
blotting paper and the flat-iron again applied to remove any slight excess of spermaceti. 
The screen now presented the appearance shown in fig. 1. 
Several screens were made and tested. The test consisted in causing a glow lamp 
on one side of the screen to balance a glow lamp placed at the same distance on the 
other side by means of a variable resistance in the circuit. The spots, if correctly 
