[Illustration: Fig. 303. No. 1. A. The electric light. B. The narrow slit through which the light passes to the convex lens, C. D. The prism. E. The spectrum. No. 2 is the same for A B C D; but F is the convex lens collecting the scattered rays, and forming white light at G.] VII. _Duration of the Impression of Light._ If a circular disc is painted with the prismatic colours taken in the same proportion with respect to each other in which they are exhibited in the spectrum made by the prism, and the wheel is turned swiftly, then the individual colours disappear, and nearly white light is apparent. The cause is due to the same principle that creates the appearance of a complete circle of fire when a burning squib is moved quickly round before it is thrown away to burst, and as it is evident that the burning squib cannot be in every part of the circle at the same moment, there must be some inherent faculty belonging to the human eye which enables it to retain for a definite period the impression of images that may fall upon it; and this principle has been so far pressed, as it were, beyond its limits, that it is gravely asserted the image of a man's murderer "might be discovered on the retina of the eye-ball if that could be examined sufficiently quick after death." The fixture of the picture is said to be due to a sort of natural photographic process; but such fanciful statements often lead the mind into dream-land only, and so we will return to the fact of the duration of the impression of light on the eye as evidenced by several ingenious optical instruments, and especially by the scientific inventions of Dr. Faraday, Dr. Paris, and of Mr. Thomas Rose of Glasgow. By careful experiment M. D'Arcy found that the light of a live coal moving at the distance of 165 feet, maintained its impression on the [Page 312] retina during the seventh part of a second. Hence the cause of the recomposition of white light when the colours on the disc are quickly rotated. Each colour at any point succeeds the other before the impression of the last is gone from the eye, and provided the colours move round within the seventh part of a second, they are all impressed together on the eye, and meeting on the retina, produce the effect of white light. VIII. _The Phenakistiscope._