other work; and the efforts of Boyle and Hooke cleared the way for the
optical career of Newton. He conquered the difficulty which Hooke had
found insuperable, and determined by accurate measurements the
relation of the thickness of the film to the colour it displays. In
doing this his first care was to obtain a film of variable and
calculable depth. On a plano-convex glass lens (D B E, fig. 13) of
very feeble curvature he laid a plate of glass (A C) with a plane
surface, thus obtaining a film of air of gradually increasing depth
from the point of contact (B) outwards. On looking at the film in
monochromatic light he saw, with the delight attendant on fulfilled
prevision, surrounding the place of contact, a series of bright rings
separated from each other by dark ones, and becoming more closely
packed together as the distance from the point of contact augmented
(as in fig. 14). When he employed red light, his rings had certain
diameters; when he employed blue light, the diameters were less. In
general terms, the more refrangible the light the smaller were the
rings. Causing his glasses to pass through the spectrum from red to
blue, the rings gradually contracted; when the passage was from blue
to red, the rings expanded. This is a beautiful experiment, and
appears to have given Newton the most lively satisfaction. When white
light fell upon, the glasses, inasmuch as the colours were not
superposed, a series _of iris-coloured_ circles was obtained. A
magnified image of _Newton's rings_ is now before you, and, by
employing in succession red, blue, and white light, we obtain all the
effects observed by Newton. You notice that in monochromatic light the
rings run closer and closer together as they recede from the centre.
This is due to the fact that at a distance the film of air thickens
more rapidly than near the centre. When white light is employed, this
closing up of the rings causes the various colours to be superposed,
so that after a certain thickness they are blended together to white