OPTICS. 



Sir Isaac Newton concludes, that their 

 colours arise from the thinness of the 

 transparent parts of the feathers ; that 

 is, from the slenderness of the very tine 

 hairs which grow out of the sides of the 

 grosser lateral branches of the feathers. 

 The finer webs of spiders, and the 

 transparent capillary crystals of apo- 

 phyllite, mesolite, and scolezite, are 

 often so minute as to appear coloured. 

 Gold ; nd silver leaf reflect one colour 

 and transmit another; and the infu- 

 sions of various coloured woods appear 

 blue by reflected, and yellow, orange, 

 and red by transmitted light- 



6. The parts of bodies on which their 

 colours depend are denser than the 

 medium which pervades their interstices, 



This will appear from the consider- 

 ations, that the colour of a body de- 

 pends not only upon the rays incident 

 perpendicularly on its parts, but also on 

 those incident at all other angles ; and 

 that a very little variation of obliquity 

 changes the reflected colour when the 

 thin plate, or particle, is rarer than the 

 surrounding medium ; insomuclr that 

 such a small particle will, at different 

 oblique incidences, reflect all sorts of 

 colours in so great a variety, that the 

 colour resulting from them all, con- 

 fusedly reflected from a heap of such 

 particles, will be a greyish white. 

 Whereas, if the thin plate or particle is 

 much denser than the surrounding me- 

 dium, the colours are so little changed by 

 the variation of obliquity, that the rays 

 which are reflected least obliquely may 

 predominate over the rest; so much, 

 as to cause a heap of such particles 

 to appear intensely of their colour. 



7. The size of the component parts of 

 natural bodies may be conjectured from 

 their colours. 



In order to do this we must suppose 

 that the particles have a given retractive 

 power, such as that of air, water, glass, 

 or diamond. For example, it a body 

 which is likely to have the same re- 

 fractive power as glass reflects a green 

 of the third order, its thickness will be 

 found, by the Table in page 35, to be 16 

 millionths of an inch. The difficulty 

 however consists in determining to which 

 order the particular colour belongs. 

 The following rules are given by New- 

 ton. 



Scarlets and other reds, oranges, and 

 yellows, are most probably of the second 

 order, if they are pure and intense. 

 Those of the first and third order may be 

 pretty good, only the yellow of the first 



order is faint, si 



and the orange 



order have a 



blue. The red of different 



belongs to the third order. 



Purest greens are of the third order, 

 though there may be good ones of the 

 fourth order. The greens of all vege- 

 tables seem to belong to the third order ; 

 for when they wither they turn to a 

 greenish yellow, or to a more perfect 

 yellow or orange, or perhaps to a red, 

 passing through all these intermediate 

 colours. These changes may arise from 

 the exhalation of the moisture, which 

 may leave the colouring particles more 

 dense. These last colours are too full 

 and lively to be of the fourth order, and 

 consequently the green through which 

 they have passed is likely to be of the 

 third order. 



The best blues and purples are of the 

 third order, though some of them may 

 be of the second. The colour of violets 

 Newton considers as of the third order, 

 because acids change the syrup of vio- 

 lets into a fine red, and alkalis into a 

 beautiful green. 



The azure colour of the purest and 

 most transparent sky he supposes to be 

 of the first order, and to arise from par- 

 ticles of vapour before they have at- 

 tained the size requisite to reflect other 

 colours. 



The most intense and luminous 

 whites are of the first order, and those 

 which are less strong and luminous are 

 a mixture of colours of several orders. 

 Of this last kind is the whiteness of 

 froth* paper, linen, and most white 

 substances. The colour of white metals 

 seems to be of the first order. The 

 colours of gold and copper are of the 

 second or third order. The colour of 

 mercury is probably of the first order. 



Blackness requires a smaller size of 

 particles than any colour, for at all 

 greater sizes there is too much light 

 reflected to constitute this colour. If 

 the particles are a little less than what is 

 necessary to reflect the white and very 

 faint blue of the first order, they will 

 reflect so very little light as to appear 



* We have retained froth in tliis list, though we 

 think it demonstrable ihat/roM owes its wh-.u-iie^s, 

 like the powder or minute parts of all transparent 

 bodies, to its reflecting an immense number of i.nages 

 of the luminous objects above and around it. In the 

 open air each of the little spherical vesicles or bubbles 

 of which froth is composed, reflects an image of the 

 sky, and all these accumulated images con>titute it> 

 white colour. Pounded glass, snow, and other in>dies 

 which are transparent, owe their whiteness to the 



