pressure is removed. The smallest ring of a set may thus be in- 

 creased to double or triple its former diameter. But to produce that 

 which may properly be called contact, mere pressure is not sufficient; 

 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 rings is 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, a 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 sur- 

 rounding 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 subdi- 

 vided, and blending with the green edge, surrounds it with yellow, 

 orange, and red. 



The order of the colours, whether the rings are seen by re- 

 flection 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 re- 

 ference 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. 



