54 



be done by separating their parts by 

 mechanical division ; or the subtle per- 

 vading; agent may be banished by heat 

 or chemical action. Thus, pulverised 

 glass and the shavings of horn are no 

 longer transparent. Wet paper becomes 

 opaque upon evaporating the moisture 

 by heat ; and salts and other substances, 

 held in solution, lose their transparency 

 by precipitation. 



On such grounds Newton assumes 

 that the parts of opaque bodies are se- 

 parated by interstices ; but he even goes 

 further. He affirms that these inter- 

 stices must have certain determinate 

 magnitudes, and there are limits within 

 which they cannot come. In the 

 experiment with the lenses (54), the 

 black central spot, within which the 

 transmission of the light was total, was 

 much larger than the point of contact 

 of the glass, and no light was reflected 

 until the air attained a certain thickness. 

 In like manner, if a soap-bubble be 

 examined, it will be found that as the 

 liquid which forms it subsides, the top 

 becomes so thin as to reflect no light, 

 and, consequently, appears black. In 

 order, therefore, to reflect light, the dis- 

 tances between the particles of a body 

 must be such as to allow the pervading 

 medium, whatever it be, to exist in suffi- 

 ciently thick parts ; otherwise the dis- 

 tances between the particles would be 

 less than half the interval of the fits, and 

 the ray, throughout the whole thickness 

 of the medium, would be in a fit of easy 

 transmission. 



This Newton considers to be the case 

 with water, glass, translucid stones, 

 and other transparent substances. That 

 there are spaces between their particles 

 is beyond doubt. But those spaces, re- 

 latively to the pervading medium, are 

 circumstanced in the same way as the 

 air immediately around the point of con- 

 tact of the lenses ; they are too small to 

 reflect light, and the ray never has space 

 enough in any of them to pass from a 

 fit of easy transmission into one of easy 

 reflection, and consequently it is wholly 

 transmitted. 



(74.) This theory derives some further 

 support from the effects of the solution 

 of solid substances in liquid. The par- 

 ticles, by a subdivision, inconceivably 

 minute, are brought into closer con- 

 tact ; and, although the solid after 

 solution, fills a greater space, and the 

 aggregate of the interstices of its parts 

 must therefore be increased, yet the 

 subdivision is so minute that every sin* 



A POPULAR ACCOUNT 



gle interstitial space is very much dimi- 

 nished. This diminution is carried so 

 far, that it is less than the interval be- 

 tween a fit of easy transmission and one 

 of easy reflection, and consequently the 

 light is transmitted, and the solution is 

 diaphanous. 



(75.) There are many analogies which 

 support the opinion, that the colours of 

 natural bodies are produced upon the 

 same principle, governed by the same 

 laws, and attended with effects in all 

 respects the same as those produced by 

 their transparent plates, as explained 

 in the last chapter. One of these thin 

 transparent plates, provided it be of uni- 

 form thickness, will, in every part, re- 

 flect the same colour. Let it be slit into 

 threads, or broken into fragments ; every 

 piece will separately reflect the same 

 colour as the entire plate. A heap of 

 these fragments will still appear of the 

 same colour ; and if a natural body be 

 considered as amass of such fragments, 

 it must, on the same grounds, exhibit 

 the same colours. 



It will be recollected, that thin plates, 

 viewed at different obliquities, shifted 

 their tints, the length of the fits, and, 

 therefore, the species of the light re- 

 flected, changing with every change in 

 the obliquity of the light to the plate. 

 In conformity with this, many natural 

 bodies vary their hues with the point 

 from which they are viewed. The plu- 

 mage of birds, and more especially of 

 the peacock, presents a splendid in- 

 stance of this. Changeable silks, and 

 almost all dyed clothes, are attended 

 with a similar effect. A spider's web, 

 when finely spun, exhibits colours. All 

 these phenomena bear a close and ob- 

 vious analogy to those which occupied 

 our attention in the last chapter. 



(76.) But, further, some substances 

 reflect one colour and transmit another, 

 like the air between the lenses in (54). 

 Examples of this will be found in leaf 

 gold, some species of stained glass, and 

 the infusion of lignum nephriticum. 

 The coloured powders used by painters 

 change their tints by mere grinding. 

 The parts being thus reduced in size, 

 reflect various colours, in the same 

 manner as a transparent plate would by 

 reducing its thickness. 



No example more strongly supports 

 Newton's reasoning than those which 

 are so common in chemical experiments, 

 in which various changes of colour are 

 produced by the admixture of different 

 species of liquids. Two liquids, each of 



