pressure is removed. The smallest ring of a set may thus be increased to double or triple its former diameter. But to produce that
which may properly be called contact, mere pressure is not suflicient;
and it will be necessary to give a little motion laterally backwards
and forwards, accompanied with moderate pressure.
The number of the rings, which may be seen at once, varies from eight or ten to as many as twenty, accordingly as the light is less or more favourable. As the size of the ringsis altered, so the colours of them are much affected by pressure. When a convex surface, of fifteen feet radius, is laid upon a plain surface, if the colour which first appears be red, 9. moderate pressure will convert it into a ring of red, with a green centre; and in the same manner, by increase of pressure, the green will give place to red; and so alternately for six or seven times, till at last, in absolute contact, the centre becomes black, surrounded by white.
The twelfth section describes the successive development of all the prismatic colours, by using lenses of greater radii. For though a small lens, of two inches, shows nothing but black and white in the series of rings that surround the centre of absolute contact, with a lens of four inches a faint red colour begins to appear in the outward rings; and this redness will be more manifest with radii of five, six, and seven inches: but the rings will not assume a green colour till a lens is used of sixteen, eighteen, or twenty inches: but it must be observed, that this and other colours appear soonest when the lens is not kept in such contact as to give a black centre.
With a lens of twenty—six inches, violet, indigo, or blue, may first be discerned at the centre. With one of thirty-four, the White surrounding the black inclines to yellow; with forty-two or forty-eight, yellow rings become visible; with fifty-nine, blue rings are plainly visible; with ten feet, orange may be distinguished from yellow, and indigo from blue; with fourteen feet, violet becomes visible.
‘When the Huygenian lens, of 122 feet, is well settled, the central spot, which in small lenses appeared black, is diluted, and drawn out into violet, indigo, and blue, surrounded with an admixture of green; while the white ring that surrounded the black spot is also subdivided, and blending with the green edge, surrounds it with yellow, orange, and red.
The order of the colours, whether the rings are seen by reflection or transmission, is such, that the most refrangible of each ring are toward the centre; but the black of one set corresponds in position to the white of the other, and the red to the green, so that the dimensions of rings, of the same colour, in each are not alike.
Hence a sudden change of colours may be produced, in each set, by intercepting that light by which they were before seen, and occasioning them to be seen by the opposite; and this alteration of colour is accompanied with an immediate change of size.
In several of the succeeding sections Dr. Herschel explains, by reference to figures, the courses of the rays by which each appearance is seen, and refers them each to the surface from which they are reflected.
