74 DISSERTATION SECOND. [mini. 



But, in explaining the permanent colour of bodies, this 

 difficulty always presents itself, — Suppose that a body re- 

 flects red or green light, what is it that decomposes the 

 light, and separates the red or the green from the 

 rest ? Refraction is the only means of decomposing light, 

 and separating the rays of one degree of refrangibility 

 and of one colour, from those of another. This appears 

 to have been what led Newton to study the colours pro- 

 duced by light passing through thin plates of any transpa- 

 rent substance. The appearances are very remarkable, and 

 had already attracted the attention, both of Boyle and of 

 Hook, but the facts observed by them remained insulated 

 in their hands, and unconnected with other optical phe- 

 nomena. 



It probably had been often remarked, that when two trans- 

 parent bodies, such as glass, of which the surfaces were con- 

 vex in a certain degree, were pressed together, a black spot 

 was formed at the contact of the two, which was surround- 

 ed with coloured rings, more or less regular, according to the 

 form of the surfaces. In order to analyse a phenomenon that 

 seemed in itself not a little curious, Newton proposed to 

 make the experiment with surfaces of a regular curvature, 

 such as was capable of being measured. He took two object 

 glasses, one a plano-convex for a fourteen feet telescope, 

 the other a double convex for one of about fifty feet, and 

 upon this last he laid the other with its plane side downwards, 

 pressing them gently together. At their contact in the cen- 

 tre was a pellucid spot, through which the light passed with- 

 out suffering any reflection. Round this spot was a coloured 

 circle or ring, exhibiting blue, white, yellow, and red. This 

 was succeeded by a pellucid or dark ring, then a coloured 

 ring of violet, blue, green, yellow, and red, all copious and 

 vivid except the green. The third coloured ring consisted 

 of purple, blue, green, yellow, and red. The fourth consist- 



