CHROMATICS. 



transparent substances, particularly soap- 

 water blown into bubbles, exhibited vari- 

 ous colours, according to their thinness ; 

 and yet, when they have a considerable 

 degree of thickness, they appear colour- 

 less. And Sir Isaac himself had observ- 

 ed, that as he was compressing two 

 prisms hard together, in order to make 

 their sides (which happened to be a little 

 convex) to touch one another, in the place 

 of contact they were both perfectly trans- 

 parent, as if there had been but one con- 

 tinued piece of glass : but round the point 

 of contact, where the glasses were a little 

 separated from each other, rings of differ- 

 ent colours appeared. And when he af- 

 terwards, farther te elucidate this matter, 

 employed two convex glasses of teles- 

 copes, pressing their convex sides upon 

 one another, Ive observed several series 

 of circles or rings of such colours, differ- 

 ent, and of various intensities, according 

 to their distance from the common cen- 

 tral pellucid point of contact. 



As the colours were thus found to vary 

 according to the different distances be- 

 tween the glass plates, Sir Isaac conceiv- 

 ed that they proceeded from the different 

 thickness of the plate of air intercepted 

 between the glasses ; this plate of air be- 

 ing, by the mere circumstance of thinness 

 or thickness, disposed to reflect or trans- 

 mit the rays of this or that particular co- 

 lour. Hence, therefore, he concluded, 

 that the colours of all natural bodies de- 

 pend on their density, or the magnitude 

 of their component particles : and hence 

 also he constructed a table, in which the 

 thickness of a plate necessary to reflect 

 any particular colour was expressed in 

 the millionth parts of an inch. 



From a great variety of such experi- 

 ments, and observations upon them, our 

 author deduced his theory of colours. 

 And hence it seems that every substance 

 in nature is transparent, provided it be 

 made sufficiently thin ; as gold, the dens- 

 est substance we know of, when reduc- 

 ed into thin leaves, transmits a bluish 

 green light through it. If we suppose any 

 body, therefore, as gold, for instance, to 

 be divided into a vast number of plates, 

 so thin as to be almost perfectly transpa- 

 rent, it is evident that all, or the great- 

 est part of the rays, will pass through the 

 upper plates, and when they lose their 

 force will be reflected from the under 

 ones. They will then have the same num- 

 ber of plates to pass through which they 

 had penetrated before ; and thus, accord- 

 ing to the number of those plates through 

 which theyave obliged to pass, the object; 



VOL. III. 



appears of this or that colour, just as the 

 rings of colours appeared different in the 

 experiment of the two plates, according 

 to their distance from one another, or the 

 thickness of the plate of air between them. 



This theory of the colours has been il- 

 lustrated and confirmed by various expe- 

 riments, made by other philosophers, 

 Mr. Delaval produced similar effects by 

 the infusions of flowers of different co- 

 lours, and by the intimate mixture of the 

 metals with the substance of glass, when, 

 they are reduced to very fine parts ; the 

 more dense metals imparting to the 

 glass the less refrangible colours, and the 

 lighter ones those colours that are more 

 easily refrangible. Dr. Priestley and Mr. 

 Canton, also, by laying very thin leaves or 

 slips of the metals upon glass, ivory, 

 wood, or metal, and passing an electrical 

 stroke through them, found that the 

 same effect was produced, viz. that they 

 were tinged with different colours, ac- 

 cording to the distance from the point of 

 explosion. 



Mr. Delaval has given also an account 

 of some experiments made upon the per- 

 manent colours of opaque substances, 

 which may prove of great importance in 

 the arts of dying, &c. 



The changes of colour in permanently 

 coloured bodies, he observes, are pro- 

 duced by the same laws that take place 

 in transparent colourless substances; and 

 the experiments by which they are inves- 

 tigated consist chiefly of various methods 

 of uniting the colouring particles into 

 larger masses, or dividing them into 

 smaller ones. Sir Isaac Newton made his 

 experiments chiefly on transparent sub- 

 stances ; and in the few places where Ite 

 treats of others, he acknowledges his 

 want of experiments. He makes the fol- 

 lowing remark, however, on those bo- 

 dies which reflect one kind of light and 

 transmit another, viz. that if these glasses 

 or liquors were so thick and massy that 

 no light could get through them, he ques- 

 tioned whether they would not, like 

 other opaque bodies, appear of one and 

 the same colour in all positions of the eye ; 

 though he could not yet' affirm it 

 from experience. Indeed it was the opi- 

 nion of this great philosopher, that all 

 coloured matter reflects the rays of light, 

 some reflecting the more refrangible 

 rays most copiously, and others those, 

 that are less so ; and that this is at once 

 the true and only reason of these colours. 

 He was likewise of opinion, that opaque 

 bodies reflect the light from their ante- 

 rior surface, by some power of the body, 

 evenly diffused over and external te it. 



