OF NEWTON'S OPTICS 



41 



manner gradually melted, assuming tints 

 more and more dilute, until it became 

 absolutely white. This white ring was 

 again bordered by a yellow one, which 

 in its turn was tinted off in a red ring. 



In this series of coloured circles the 

 blue of the first circle was very faint ; 

 the white of the second was brilliant. 

 The gradual tints assumed by the yellow 

 of the third ring in passing into the red 

 of the fourth, produced between them 

 an orange ring. 



A second series of rings succeeded, 

 the first of which, surrounding the red 

 ring of the last series, was violet, after 

 which appeared in regular succession 

 four other rings of blue, green, yellow 

 and red. In this series the green was 

 yellowish, the yellow brilliant, and the 

 red partaking of a crimson hue. 



After this came a third series. The 

 first ring in this series surrounding the 

 red of the last was purple, which was 

 regularly succeeded by four rings of 

 blue, green, yellow and red. Of these 

 the green was brisk and copious, being 

 a rich grass green, and the yellow was 

 particularly splendid ; but the red had 

 a more faded appearance, and partook 

 more of the pink and crimson, than the 

 vermilion. 



After these succeeded a fourth series, 

 consisting of two colours, green and red. 

 The green, in passing into the red, ex- 

 hibited a yellowish pink hue. Then suc- 

 ceeded three other series, each consisting 

 of two rings ; the inner ones being various 

 shades of green, and the outer ones 

 various shades of red ; each of the colours 

 become more and more dilute, as the 

 diameters of the rings increased. 



(53.) Due consideration of these phe- 

 nomena suggested some very important 

 conclusions. Of the light which pene- 

 trated the glass within the central black 

 circle none was reflected, for in that case 

 the circle would take the colour of the 

 reflected light. The incident light was, 

 therefore, in this case, either stifled and 

 absorbed by the glass, or was transmit- 

 ted. To ascertain this, the eye was 

 placed behind the lenses, so as to receive 

 the transmitted light. The central spot 

 now appeared white, proving that all the 

 light incident on the glass was here 

 transmitted. Again, of the light incident 

 upon the first ring of the first series, the 

 rays composing a bluish colour alone 

 were reflected. The remaining rays were 

 transmitted, as appeared by viewing the 

 lenses on the other side ; the colour of 

 the first ring being that which was com- 



plemental to its reflected colour, or that 

 which combined with the reflected colour 

 would produce white. In the same way, 

 each ring of each series was found to 

 transmit the colour complemental to that 

 which it reflected, which was proved by 

 viewing the light through the lenses. 



These phenomena were attended with 

 many circumstances, which rendered it 

 probable that some connexion subsisted 

 between the colours of the reflected and 

 transmitted light, and the thickness of 

 the air-lens, at the place where these 

 colours were produced. The same colour 

 was observed to be arranged in a circle 

 round the centre of the lens. It was 

 evident, that in all parts of such a circle 

 the thickness of the air-lens was the 

 same. Again, in passing from one con- 

 centric circle to another, the tint was ob- 

 served to undergo a change. In dif- 

 ferent concentric circles the thickness of 

 the air-lens was different. Here, then, 

 were two important steps towards the 

 discovery of a connexion between the 

 colour of the light, and the thickness of 

 the air, which reflected or transmitted it. 

 By pressing the glass lenses together, 

 so as to force them into closer contact, 

 the diameter of each circle, at which the 

 air-lens had a given thickness, would ob- 

 viously be increased. If it were true, 

 that the colour of the reflected light de- 

 pended on the thickness of the air at the 

 points of reflexion, it would follow, that 

 upon pressing the glass lenses together, 

 each coloured circle would be enlarged. 

 It was accordingly found, that upon ap- 

 plying such pressure, the central spot 

 was increased, and each coloured circle 

 expanded its dimensions, and retreated 

 from the centre. These indications were 

 further confirmed, by pressing the lenses 

 more closely at one side of the centre 

 than at the other, the colours still re- 

 treating from the points of closest con- 

 tact. 



(54.) Aware of theheterogeneous nature 

 of solar light by previous investigations, 

 Newton considered it probable that these 

 coloured rings were not the effects of 

 one simple cause, or of a single action of 

 the transparent medium on the solar ray, 

 but conjectured that it might rather be 

 the result of the combined actions on all 

 or several of the component parts of 

 light. To simplify the phenomena, and 

 thereby facilitate the analysis, he deter- 

 mined to expose the lenses successively 

 to the different species of homogeneous 

 light, and to observe and carefully note 

 the separate effects of each. He con- 



