OF NEWTON'S OPTICS. 



37 



another as they appear in the spectrum, 

 so that, in effect, the circumference of 

 the circle will exhibit, as it were, a round 

 prismatic spectrum. By the principles of 

 mechanics, let the centres of gravity of 

 the arcs AB, B C, &c. be respectively 

 found, and let these points be r, o, y, g, 

 b, i, and v. Now, suppose that it be re- 

 quired to determine the tint which would 

 result from the mixture of red, green, 

 and blue, in certain given proportions. 

 Let circles be described round the points 

 r, g, and b, the magnitudes of which are 

 to be made proportional to the quanti- 

 ties of the three colours in the proposed 

 mixture. Let the common centre of 

 gravity of these circles be found, and let 

 it be m; and from the centre O through 

 m draw Om, to meet the circle at t. The 

 colour at the point t will be the tint 

 sought, and the line Om will represent 

 " its fullness, or intensity, that is, its 

 distance from whiteness." Thus, if t 

 should fall exactly in the middle of any 

 of the arcs, AB, B C, &c. the tint will 

 be the purest of the corresponding co- 

 lour ; but if it be distant from the middle 

 point, it will partake of the colour which 

 occupies the next arc, towards which it 

 lies. Again, if m fall on the centre O, 

 the colour will be, as it were, infinitely 

 diluted, and will be a perfect white ; but, 

 on the other hand, the nearer m is to the 

 circumference, the more intense and 

 florid the tint will be. 



Newton conceived this method to be 

 sufficiently accurate for practice, al- 

 though not mathematically true. This 

 is a subject, however, in which much im- 

 provement has been introduced in later 

 times. It would not be to our purpose 

 here to enter upon it, our design being 

 merely to present to the reader in a po- 

 pular form a sketch of the labours of 

 Newton in the science of light. Those 

 who desire a short account of the mo- 

 dern discoveries, will find one in the ad- 

 mirable article on LIGHT, by Mr. Her- 

 schel, in the Encyclopaedia Metropoli- 

 tana. 



In applying his theory of light to ex- 

 plain the phenomena of the colours of 

 natural bodies, Newton assumes, " that 

 every body reflects the rays of its own 

 colour more copiously than the rest, and 

 derives its colour from their excess, or 

 predominance, in the reflected light." 

 When a beam of solar light falls upon a 

 violet, a decomposition immediately en- 

 sues. The red rays, and those of the less 

 refrangible character, are either trans- 

 mitted through the body, or absorbed 



and stifled ; those of the bluish, or violet 

 hue, and of the more refrangible species, 

 are copiously reflected, and produce in 

 the spectator the effect which in ordinary 

 language is denominated the violet colour 

 of the object. 



Several ingenious experiments support 

 this reasoning. A natural object, what- 

 ever be its colour, will, if placed in ho- 

 mogeneous light, take for the time the 

 colour of that light, proving thereby its 

 capability of reflecting, in some degree, 

 lights of all colours. But when it is 

 placed in homogeneous light of its own 

 colour, it will appear much more re- 

 splendent than in light of any other 

 colour. Hence we infer that it possesses 

 a capability of reflecting light of its own 

 colour more abundantly than light of 

 any other colour. Thus cinnabar, a red 

 substance, placed in homogeneous red 

 light, exhibits a splendid red; let it, 

 however, be illuminated with green or 

 blue light, and it will assume these 

 colours, but with great faintness. 



The colours of transparent liquors 

 vary with their thickness. If a red 

 liquor be poured into a glass of conical 

 or tapering shape, and held between the 

 light and the eye, it will appear of a pale 

 dilute yellow at the narrowest part of the 

 glass ; a little higher, where the glass is 

 wider, it becomes orange ; higher still it 

 becomes red ; and, finally, in the widest 

 part, exhibits a deep dark red. We 

 must, therefore, infer that a small quan- 

 tity, or thickness of the liquor, intercepts 

 a portion of the violet and indigo rays, 

 so that the remaining rays which it trans- 

 mits form a pale yellow. A greater 

 quantity of the liquor, besides stopping 

 the violet and indigo, also arrests the blue 

 rays, and a part of the green, transmit- 

 ting the other component parts of light, 

 the mixture of which produces an orange. 

 A still greater quantity of the fluid will 

 intercept all the green, and a great part 

 of the yellow, so that the transmitted 

 light approaches to a red, becoming a 

 deep dark red, when the quantity of the 

 fluid is so great as to absorb the whole 

 of the orange light. 



W^e have in this description assumed 

 several distinct effects, but the changes 

 of colour are not sudden, but take place 

 by an imperceptible gradation, an ob- 

 vious consequence of the tapering form 

 of the glass. If the glass were formed 

 of a number of cylinders rising one above 

 another, the diameter of each exceeding 

 that below it by a certain magnitude, the 

 changes of colour would be sudden and 



