158 
CAPTAIN ABNEY ON THE SENSITIVENESS 
adjusted by a screw arrangement wmrked by the milled head, C. The brass ring 
is grooved on its outer edge, and round it and a brass pulley, H, passes a thread, 
and the ring, B, can be rotated by means of an universal joint and the handle, K. 
The pulley, H, by an obvious arrangement, can be moved towards or away from the 
annulus, thus allowing the thread to be slackened or tightened as may be desired. 
When F is in position the annulus covers the slit, S. By turning the handle, K, 
which is of such a length that it can be used 5 feet off from G, different parts of the 
annulus are caused to pass across S. When in use a small glow-lamp, carefully 
shaded, is attached to L, the frame of the camera, and after an extinction the readings 
are made by passing a current through it temporarily. It might be thought that an 
easier arrangement would be to place the mounted annulus in front of the eye, but 
tlie difficulty of reading without illuminating the retina prevented its adoption, and 
the uncertainty that would exist as to the precise part of the annulus which came 
between the spot of light and the eye, owing to the large eye-hole, also prevented 
its use in that position. Perfectly unfettered vision is essential. 
3, Graduation o f Annulus. 
The annulus, when thus mounted, was ready for graduation, and as the method 
adopted is, it is believed, new, a description of it is given. The colour-patch 
apparatus described in Part II., “Colour Photometry” (‘ Phil. Trans.’), was utilized, 
but to the front of the slit in the spectrum a piece of plain Avorked glass, M’, Avas 
attached as shoAvn (fig. 2). The beam of light coming from the prism, P, after 
passage through the lens, L', and the slit, S, was partly reflected from a plain 
mirror M' on to M”, a silvered mirror, and partly transmitted through M', the ratio 
of the amount of reflected to transmitted light vaiying according to the angle 
which M’ made with the direction of the beam. The direct beam after passing 
through M' also passed through the annulus, B, and through the lens, L", forming 
:in image of the face of the prism, P, on A. The annulus, B, could be rotated so 
that the intensity of illumination of the patch of light on A could be increased 
or diminished at Avill. The reflected part of the beam also jDassed through the 
margin of the lens, L”, and formed a patch of light of the same colour and A^ery 
