286 EXPERIMENTAL PHYSIOLOGY. 



there is only one position in which the two beams of light 

 are brought to a focus. Move the retina towards the cornea, 

 and observe two images; close the right-hand hole and the 

 right-hand image disappears. Bring the retina posterior to 

 the principal focus, and again there are two images. On 

 closing the right-hand hole, the left-hand image disappears, 

 and vice versd. 



2. Mixing of Colour Sensations. 



(a.) Arrange on the spindle of the rotating apparatus the 

 disc with coloured sectors provided for you. 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. 



(6.) Arrange three of 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. 



(c.) 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. 



(d.) Arrange two coloured discs of vermilion and bluish- 

 green in the proportion 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 

 to 78-7 of the black. On rotating, a grey colour is obtained 

 from both sets of discs. 



(e.) Lambert's Method. On a black background place a 

 blue wafer or square of blue paper, and six or seven inches 

 behind it a yellow square or wafer. Hold a plate of clear 

 glass vertically, about ten inches above and midway 

 between the two squares. Look obliquely through the 



