CHROMATICS. 
gold^ for instance, to be divided into a vast 
number of plates, so tbin as to be almost 
perfectly transparent ; it is evident that all, 
or the greatest part of the rays, will pass 
through the upper plates, and wlicn they 
lose their force v.'il! be reflected from the 
under ones. They will then have the same 
number of plates to pass through which they 
had penetrated before ; and thus, accord- 
ing to the number of those plates through 
which they are obliged to pass, the object 
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 illus- 
trated and confirmed by various experi- 
ments, made by other philosophers. Mr. De- 
laval produced similar effects by the infusions 
of flowers of different colours, and by the in- 
timate mixture of the metals with the sub- 
stance of glass, when they are reduced to 
very fine parts ; the more dense metals im- 
parting to the glass the less refrangible co- 
lours, and the lighter ones tliose colours 
tliat 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, foniui 
that the same effect was produced, viz. that 
they were tinged with different coloius, 
according to the distance from the point 
of explosion. 
Mr. Delaval has given also an account of 
some experiments made upon the pernia- 
ment 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 produced 
by the same laws that take place in trans- 
parent colourless substances ; and the ex- 
periments by which they are investigated 
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 ti'ansparent substances ; and in the few 
places where he treats of others, he ac- 
knowledges liis want of experiments. He 
makes the following remark, however, on 
those bodies which refleet one kind of light 
and transmit another, viz. that if these 
glasses or liquors were so thick and massy 
that no light conld get through them, he 
questioned 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 ex- 
perience. Indeed it was the opinion of this 
great philosonher, that all coloured matter 
reflects the rays of light ; some reflecting 
tlie more refrangible rays most copiously, 
and otlicrs 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 anterior snrfiice, by some power of 
the body evenly diffused over and external 
to it. With respect to transparent coloured 
bodies, he thus expresses himself : “ A 
transparent body which looks of any colour 
by transmitted light, may also look of the 
same colour by reflected light ; the light of 
that colour being reflected by the farther 
surface of that body, or by the air beyond 
it : and then the reflected colour will bG 
diminished, and perhaps cease, by making 
the body very thick, and pitching it on the 
back -side to diminish the reflection of its 
farther surface, so that tile light reflected 
from the tinging particles may predominate. 
In such cases the colour of the reflected 
light will be apt to vary from that of tlm 
light transmitted.” 
To search out the truth of tliese opinions, 
Mr. Delaval entered upon a course of ex- 
periments with transparent coloured liquors^ 
and glasses, as well as with opaque and 
semitransparent bodies. And from these 
experiments he discovered several remark- 
able properties of the colouring matter; 
particularly, that in transparent coloured 
substances it does not reflect any light; and 
when, by intercepting tlie light which was 
transmitted, it is hindered from passing 
through such substances, they do not vary 
from their former colour to any otlier, but 
become entirely black. 
This incapacity of the colouring particles 
of transparent bodies to reflect light, being 
deduced from very mimeroBs experiments, 
may therefore be taken as a general law. 
It will appear the more extensive if it be 
considered that, for the most part, the ting- 
ing particles of liquors, or other transpai-ent 
substances, are extracted from opaque 
bodies; that the opaque bodies owe their 
colours to those particles in like manner as 
the transparent substances do ; and iliat by 
the loss of them, they are deprived- of their 
colours-. 
Notwithstanding these and many other 
experiments the theory of colonr seems not 
yet determined with certaintyi The d»» 
