XXXV111 



THE PROGRESS OF 



tent of the arch, the breadth of the coloured 

 bow, the position of the secondary bow, it* dis- 

 tance from the primary, and by explaining the 

 inversion of the colours. 



As all colour conies from the rays of light, we 

 must conclude that the colour of every particular 

 body is owing to its reflecting the particular kind 

 of ray of which it exhibits the colour. If you 

 cast upon a coloured body a homogeneous ray of 

 light, it appears much more vivid ; that is, it re- 

 flects much more light when the ray cast upon it 

 is of its own colour. Transparent bodies some- 

 times reflect one kiud of ray, and transmit 

 another. 



But here a difficulty occurs. Suppose a body 

 to reflect red or green light, what is it that de- 

 composes the light, and separates the red or the 

 green from the rest ? It was this difficulty that 

 led Newton to study the colours produced by 

 light passing througli thin plates of any trans- 

 parent substance. When two transparent bodies, 

 such as pieces of glass having their surfaces 

 somewhat convex, are pressed together, a black 

 spot is formed at the contact of the two, which 

 is surrounded with coloured rings, more or less 

 regular, according to the form of the surfaces. 

 To analyse this phenomenon, Newton made ex- 

 periments with surfaces of i regular curvature, 

 capable of being measured. He took two object 

 glasses, one a plane 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 undermost, pressing 

 them gently together. At their contact, in the 

 centre was a pellucid spot, through which the 

 light passed without suffering any reflection. 

 Round this spot was a coloured ring, exhibiting 

 blue, white, yellow, and red. This was suc- 

 ceeded 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 con- 

 sisted of green and red ; those that succeeded 

 became gradually more dilute, and ended in 

 whiteness. It was possible to count as far as 

 seven. 



The colours of these rings were so marked by 

 peculiarities in shade and vivacity, that Newton 

 considered them as belonging to different orders ; 

 o that an eye accustomed to examine them, on 

 any particular colour of a natural object being 

 pointed out, would be able to determine to which 

 order in this series it belonged. 



Thus we have a system of rings or zones sur- 

 rounding a dark central spot, and themselves 

 alternately dark and coloured ; that is, alternately 

 transmitting the light, and reflecting it. It is 

 evident, that the thickness of the plates of air in- 



terposed between the glasses, at each of those 

 rings, must be a material element in the arrange- 

 ment of this system. New ton, therefore, under- 

 took to compute their thickness. Having care- 

 fully measured the diameters of the first six 

 coloured rings, at the most lucid part of each, he 

 found their squares to be as the progression of 

 the odd numbers 1, 3, 5, 7, &c. The squares of 

 the distances, from the centre of the dark spot to 

 each of these circumferences, were therefore in 

 the same ratio ; and, consequently, the thickness 

 of the plates of air, or the intervals between the 

 glasses, were as the numbers 1, 3, 5, 7, &c. 

 \\ lien the diameters of the dark or pellucid 

 rings, which separated the coloured rings, were 

 measured, their squares were found to be as ihc 

 even numbers 0, 2, 4, 6 and, therefore, the 

 thickness of the plates, through which the light 

 was wholly transmitted, were as these numbers. 



The curvature of the convex glass being the 

 thickness of the plates of air which corresponded 

 to the different rings, was computed. An inch 

 being divided into 178,000 parts, the thickness of 

 the air for the first series, or for tho luminous 



""& wa8 TTS55, n^To, Hfiioo' &C ' inch ' For the 

 second series -JL, ^ &c. inch. 



When the rings were examined, by looking 

 through the lenses in the opposite direction, the 

 central spot appeared white, and in other rings 

 red was opposite to blue, yellow to violet, and 

 green to a compound of red and violet the 

 colours formed by the transmitted and reflected 

 light being what are now called complimentary, 

 or nearly so, of one another ; that is, such as 

 when mixed produce white. 



When the fluid between the glasses was differ- 

 ent from air, as when it was water, the succession 

 of rings was the same ; the only difference v. a, 

 that the rings themselves were narrower. 



When experiments were made on thin plates 

 in such a way that the plate was denser than the 

 surrounding medium, as in the case of soap 

 bubbles, the same phenomena were observed to 

 take place. 



To the different degrees of density, then, in 

 transparent bodies, there seemed to be attached 

 the powers of separating particular colours from 

 the mass of light, and of rendering them visible, 

 sometimes by reflection, and in other cases by 

 transmission. Now r , as there is reason to think 

 that the atoms of bodies are transparent, they 

 may be conceived to act on light after the man- 

 ner of thin plates, and to produce, each accord- 

 ing to its thickness and density, its appropriate 

 colour, which therefore becomes the colour of 

 the surface. 



But these experiments led to other conclusions, 

 new and unexpected. It was impossible to ob- 

 serve, without wonder, the rings alternately 



