3. _Straight lines appear interrupted._ If a ruler is held between the eye and the light of a bright lamp or the sun, we perceive a very distinct hollow on the edge of the ruler in the part corresponding to the light. When one point of the retina is affected by a light which undergoes periodical and regular variations, the duration of the period being sufficiently short, there results a continuous impression, like that which would be produced if the light given during each period were distributed in an equal manner throughout the whole duration of the period. To verify the truth of this law, we will make use of some discs, such as that represented in fig. 105. The innermost circle is half white and half black; the middle circle has two quarters, or half its periphery, white, and the outer circle has four eighths’ white, the rest being black. If such a disc is turned round, its entire surface will appear grey; only it is necessary to turn it with sufficient force to produce a continuous effect. The white may also be distributed in other ways, and provided only that on all the circles of the disc the proportion of the angles covered with white is the same, they will always exhibit the same grey colour. Instead of black and white we may make use of different colours, and obtain the same resultant colour from all the circles, when the proportion of the angles occupied by each of the colours in the different circles is the same. [Illustration: Fig. 105.—Disc which appears uniformly grey by reason of its rotation.] If we paint on a disc a coloured star, which is detached from a foundation of another colour (fig. 106), during the rapid rotation of the disc the centre affects the colour of the star; the outer circle assumes that of the background, and the intermediate parts of the disc present the continuous series of the resultant colours. These results are in accordance with the theory of the mixture of colours. [Illustration: Fig. 106.—Disc with a star painted on the background of another colour.] Rotative discs, which are so much used in experiments in optical physiology, were employed for the first time by Müsschenbroeck; the most simple is the top. M. Helmholtz ordinarily uses a brass spinning-top, which fig. 107 represents at a third the natural size. It is set in motion by the hand, and its quickness may be increased or moderated at will; but it cannot be made to spin quicker than six rounds in a second; this motion will be kept up for three or four minutes. Thus, with a feeble movement of rotation, a uniform luminous impression can only be obtained by dividing the disc into four or six sections, on each of which we repeat the same arrangement of colours, light, and shade. If the number of repetitions of the design is less, we obtain, with a bright light, a more or less shot-coloured disc. [Illustration: Fig. 107.—M. Helmholtz’s top for studying the impression of light on the retina.]