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CAPTAIN W. DE W. ABNEY ON THE TRANSMISSION 
E, E, are rajs coming from a heliostat, and a solar image is formed bj a lens L 1 on 
the slit Sj of the collimator C. The parallel ra js produced bj the lens L., are partiallj 
refracted and partiallj reflected. The former pass through the prisms P l5 P 2 , and are 
focussed to form a spectrum bj a lens L 3 on D, a moveable screen, in which is a slit S 2 . 
The rajs coming through S 3 are collected bj a lens L 4 to form a monochromatic image 
at F of the near surface of the second prism. When D is removed the image of the 
surface of the prism is white, obtained bj tilting the lens L 4 at an angle as shown. 
The reflected rajs from P x fall on G, a silver-on-glass mirror. Thej are collected 
bj L 5 , and form a white image of the prism, also at F. 
At M is a small electromotor carrjing a disc with moveable sectors, so that anj 
amount of incident light maj be cut off. 
Bj this arrangement both the comparison-light and the spectrum itself are formed 
bj the light coming through the slits. 
§ I. Graduation of the Sectors. 
As the intensitj of the comparison-light can onlj be altered bj opening or closing 
the sectors of the rotating discs, it is essential that their graduation shall be good. As 
a matter of fact, the circles were carefullj divided, but the graduation was onlj regarded 
as sufficientlj accurate when the aperture used was more than 5°, since for verj small 
angles the ratio of anj small error in graduation to the aperture used might then 
not be negligible, which it would be where the aperture was fairlj large. As it almost 
alwajs happened that coloured light had to be measured, which was verj much 
dimmer than the comparison-light diminished bj an aperture of 5° in the discs, it 
became necessarj to adopt some other means. To accomplish this, one of the two 
following methods was emplojed: 1st, either the light was absorbed bj means of 
coloured glasses ; or 2nd, a mirror of unsilvered glass was used. Both of these plans 
enabled readings of coloured light of feeble intensitj to be measured with apertures of 
the rotating discs, commencing at 90°. With such modified reflected light it was 
rarelj necessarj to close the sectors to less than 5°. The glasses used were red, green, 
blue, or violet, repeated experiment having shown to General Festing and mjself 
that it makes no difference in the result what colour the comparison-light maj be. 
In everj case the ratio of the diminished intensitj to the original intensitj of the 
comparison-light was determined, and the measures of the former reduced to the scale 
of the latter. It has been almost a surprise to find what great accuracj can be 
attained in reading bj the new method. It is a matter of the greatest ease to read 
within 1 per cent, of the mean of a series of observations when the source of illumi¬ 
nation is the sun ; the coloured shadows are so steadj that the verj slightest move¬ 
ment of the slit across the spectrum is at once perceived. 
