From what has been stated in reference to the solar spectrum it will evidently appear, that white light is nothing else than a compound of all the prismatic colours; and this may be still farther illustrated by shewing, that the seven primary colours, when again put together, recompose white light. This may be rudely proved for the purpose of illustration, by mixing together seven different powders, having the colours and proportion of the spectrum; but the best mode, on the whole, is the following. Let two circles be drawn on a smooth round board, covered with white paper, as in fig. 34: Let the outermost be divided into 360 equal parts; then draw seven right lines as A,B,C, &c., from the center to the outermost circle, making the lines A and B include 80 degrees of that circle. The lines B and C, 40 degrees; C and D, 60; D and E, 60; E and F, 48; F and G, 27; G and A, 45. Then between these two circles paint the space AG red, inclining to orange near G; GF orange, inclining to yellow near F; FE yellow, inclining to green near E; ED green, inclining to blue near D; DC blue, inclining to indigo near C; CB indigo, inclining to violet near B; and BA violet, inclining to a soft red near A. This done, paint all that part of the board black which lies within the inner circle; and putting an axis through the centre of the board, let it be turned swiftly round that axis, so that the rays proceeding from the above colours, may be all blended and mixed together in coming to the eye. Then the whole coloured part will appear like a white ring a little grayish--not perfectly white, because no art can prepare or lay on perfect colours, in all their delicate shades, as found in the real spectrum. [Illustration: _figure 34._] That all the colours of light, when blended together in their proper proportions, produce a pure _white_ is rendered certain by the following experiment. Take a large convex glass, and place it in the room of the paper or screen on which the solar spectrum was depicted (LM fig. 31), the glass will unite all the rays which come from the prism, if a paper is placed to receive them, and you will see a circular spot of a pure lively white. The rays will cross each other in the focus of the glass, and, if the paper be removed a little further from that point, you will see the prismatic colours again displayed, but in an inverted order, owing to the crossing of the rays. SECT. 2.--ON THE COLOURS OF NATURAL OBJECTS.