OF THE RETINA TO LIGHT AND COLOUR. 
157 
only obstruction but also absorption. As the particles were black and very opacjue 
the light penetrating, and which was reflected from the surfaces of the particles, 
would be colourless, or rather uncoloured. A photographic plate sensitive to all 
visible rays was exposed to the spectrum, and on the same plate, and just below 
it, a more prolonged exposure was given after its passage through a portion of the 
annulus, and on development the two images appeared identical from B to, at 
all events, near G, but beyond the image in the extreme violet was rather less 
opaque in the latter than in the former, indicating a slightly increased absorption for 
those rays. The use of this annulus was therefore possible and easy between B and 
G, and between these limits only has it been emplo 3 md. It seems probable that this 
falling off in the extreme violet may be due to scattering by fine particles. The 
following is the method adopted to mount the annulus :—A hole was pierced in the 
glass exactly at the centre of the two circles and the glass was cut into a disc 
concentric with the circles. 
The disc of glass. A, is pierced in its centre with an aperture of just sufficient size 
to allow a pin with a screw-thread springing from a brass plate attached to the 
wooden slide, G, to penetrate. The disc of glass, F, with the annulus on it, being 
also pierced at its centre with an aperture, can be passed over the same pin. The two 
discs are clamped together with a milled-headed nut, D, a washer of paper, E, being- 
placed between the two. The disc. A, is cemented into a circular ring, B, graduated 
into degrees. On A is ruled a line joining centre and the zero of the graduation. The 
junction of the most and least opaque parts of the annulus is caused to lie along that 
line. The disc, A, is mounted so that the marked line on it, when the zero point is at 
the index, M, passes across the centre of the slit, S, the width of which can be 
