Another mode of exhibiting colour blindness, and one of the first adopted, is by making mixtures of colours with rapidly rotating colour discs. In my own experiments I have chosen a red, which is scarlet, over which a wash of carmine has been brushed. It has a dominant wave-length of 6300. The green is an emerald-green, and has a dominant wave-length of 5150. The blue is French ultra-marine, with a dominant wave-length of 4700. The card discs, of some 4 inches diameter, are coated with these colours as pastes, and by making an incision in them radially to the centre, as before described, and inter-locking them, the compound disc can be caused to show sectors of any angle that may be required. Outside these are the discs of black and white, the proportions of which can be altered at will. The light thrown on the rotating sectors being that from an electric arc light, normal vision requires 118° of red, 146° of green, and 96° of blue to match a grey made up of 75 parts of white and 285 parts of black. For the last two numbers a correction has been made to allow for the small amount of white light reflected from the black surface. This correction has also been made in the subsequent matches which will be described. Colour mixtures such as these are conveniently put in the form of equations, and that given will then be shown as follows-- 118 R + 146 G + 96 U = 75 W + 285 B. (Here R, G, U, W, and B are used to indicate Red, Green, Blue, White, and Black.) This match was exact also for all the colour blind, for the deficiency in one grey is also a deficiency in the other. With a red-blind, however, very different matches can be made, as the red pigment is a complex colour. There is in it, besides red, a certain amount of yellow, whilst in the green there is, besides green, a small amount of a red and a larger amount of yellow. The yellow will not only stimulate the green sensation, but also the red where it is present. Although in complete red-blindness the red sensation is totally absent, we may expect that a mixture of red and blue, as well as of green and blue, will enable a match to be made of the grey produced by the mixture of white and black. This was the case. We have the following proportions-- 295 R + 65 U = 45 W + 315 B. When the green disc is substituted for the red, the red-blind made the following mixture-- 229 G + 131 U = 120 W + 240 B. It is worth noticing that the amount of blue in the first mixture is about half that required for the second. This tells us that the amount of green sensation stimulated in the first case is much less than in the second. As red can be substituted for green, it should follow that green, when rendered darker, should match the red. To try this a red disc replaced the black disc, and a black disc replaced the blue. The following match was then made-- 131 G + 229 B = 340 R + 20 W.