LXXIV.] COLOUR-BLINDNESS. 353 



yellow to overlap the blue, seen directly through 1 the glass ; where 

 they overlap appears white. Hering has arranged a large form of 

 this apparatus suitable for class purposes. 



(b.} Arrange on the spindle of the rotating apparatus the disc with coloured 

 sectors provided for you (fig. 275). On rotating the disc rapidly, observe that 

 it appears grey or whitish. The disc is provided with sectors corresponding 

 to the colours of the spectrum, and arranged in varying proportions. 



(c.) Arrange three of Clerk-Maxwell's colour discs red, green, and violet 

 upon the spindle of the rotating apparatus. Adjust the relative amounts 

 of these three colours, so that on rapidly rotating them they give rise to the 

 sensation of grey or white. Each disc is of a special colour, and has a radial 

 slit from the centre to the circumference. This slit enables a disc of a different 

 colour to be slipped over the other, and thus many discs can be superposed, 

 and the amount of each colour exposed regulated in any desired proportion. 



FIG. 275. Rothe's Rotatory Apparatus for Colour Discs. It is so arranged as to give 

 various rates of rotation by combining the motions of i, 2, and 3. 



(ft.) Combine a chrome-yellow disc and a blue one in various proportions, 

 and on rotating, the resultant colour is never green, but a yellowish- or reddish- 

 grey. 



(e.} Arrange two coloured discs of vermilion and bluish-green in the pro- 

 portion of 36 of the former to 64 of the latter. On the same spindle arrange 

 a white and a black disc with a diameter a little more than half that of the 

 former pair the white being in the proportion of 21.3 to 78.7 of the black. 

 On rotating, a grey colour is obtained from both sets of discs. 



4. To Test Colour-Blindness. On no account is the person 

 being tested to be asked to name a colour. In a large class of 

 students one is pretty sure to find some who are more or less colour- 

 blind. The common defects are for red and green. 



