160 Mr. A. M. Mayer on the Phenomena 



a ring of emerald-green, and this enclosed by a rose-coloured 

 ring, produced by reflexion from the interior of the tube, and 

 surrounding this ring is another of emerald-green, as shown 

 in fig. 9, in which R and R 

 are the red rings and G, G, 

 and G the circle and the two 

 rings of emerald-green. 



The hue of the rose- 

 coloured rings, as seen through 

 the tube, is much more in- 

 tense than the hue of the ring 

 viewed on the emerald ground 

 with the unassisted eye. Ex- 

 periments made with the aid 

 of an achromatized calc-spar 

 prism, and by matching on a 

 rotating disk the colours of T^-9 



the grey ring on an emerald- 

 green ground showed that the intensity of the contrast-colour, 

 as seen with the reflecting-tube, is nearly double of what 

 it is when viewed by the eye alone. The colour of the ring 

 when viewed by the unassisted eye was matched on the 

 rotator by a disk having 30 parts of Chinese vermilion + 7 

 of " Hoffman's violet R.R.R." + 34 parts of ivory-black 

 + 29 of white Bristol board. As seen through the reflect- 

 ing-tube the match appeared to be given by 57 of Chinese 

 vermilion + 15 of " Hoffman's violet R.R.R." + 28 of white 

 Bristol board. 



Another tube, 4*5 centim.in diameter and 18 centim. long, 

 gave excellent results, with two reflected rings, and on nearer 

 vision, with four reflected rings. A tube, suitable for these 

 experiments, is selected by closing one of its ends by a disk 

 of cardboard having a pin-hole at its centre. When the end 

 with the pin-hole faces the sky and we look through the other 

 end, we see bright rings caused by the reflexion of the light 

 of the pin-hole from the interior surface of the tube. If 

 these rings are circular and concentric, the tube is of circular 

 section and its axis is a right line. 



The value of this mode of observation is shown by the 

 following experiments : — Place the ring of grey paper, or a 

 circle of grey paper, about 1J centim. in diameter, on a sur- 

 face painted with cobalt-blue, not too saturated. The grey 

 ring or circle appears of an orange tint. Place a similar grey 

 ring or circle on a surface lightly washed with emerald-green, 

 it appears pinkish. The difference between the hues of the 

 ring when viewed on the cobalt-blue and on the emerald- 



