A very curious practical illustration of this may be given. Everybody has a yellow spot, more or less marked, on the retina of the eye; and this yellow spot absorbs some of the greenish-yellow rays of the spectrum. If you throw on the screen a circle of light, coloured by passing the rays from the electric lamp through chloride of chromium, the disc will then consist chiefly of red rays mixed with the rays which are absorbed by the yellow spot. If the observers in the darkened theatre look at the disc and wink slowly, they will see, with more or less distinctness, an illusion like red clouds floating over the disc, in consequence of most of the rays other than the red being absorbed by the yellow spots in their eyes.
THE MAGIC TEMPLE.
Trace on the hexagonal ground-plan, A, B, C, D, E, F, which serves as base to the building, the six semi-diameters, G A, G B, G C, G D, G E, G F, and on each of them rear perpendicularly two plain mirrors, joining all exactly at the centre, G. These glasses should be very thin, set back to back in each pair, and cut with a double bevel where the point of junction falls.
Fig. 124.
Ornament the six corners of the outer edge of the structure with as many columns and their bases, into which the outer edge of the mirrors fit by their grooves. Make the roof in any fashion you please.
In each of the six triangular spaces comprised between every two of the glass walls little pasteboard figures in relief, representing six different subjects, which will have a pleasing effect in a hexagonal form.
With some ornament harmonizing with the temple hide the junction of the mirrors.
Action.—When any one looks into one of the six openings between any two of the columns, the object there will be repeated six times, which will be an extraordinary illusion if the subjects are suitable to the arrangement of the mirrors.