420. This hypothesis, founded solely on the me- 
chanical constitution of bodies, has, however, beer* 
warmly impugned by Dr Bancroft, who contends 
that the phenomena, exhibited by the coloured rings 
in Newton's experiments, do not warrant the conclu- 
sions drawn from them ; for that the same colours oc- 
curred, and were repeated over and over again, at very 
great diversities of thickness ; so that thickness could 
not be the only cause of these repeated variations of 
colour. And, indeed, they are to be explained, he 
adds^ in the same manner as the colours of the prism 
and the rainbow, and ought not to have been employ- 
ed to explain the permanent colours in different sub- 
stances *, 
421. On Mr Delaval's attempts, to shew that the 
different colours of animal and vegetable substances 
depend on an increase or diminution in the size of 
their particles, Dr Bancroft remarks, that, instead of 
employing mechanical means, which alone ought to 
have been used in his experiments, he has recourse 
to chemical agents, which change the composition of 
bodies, and produce effects different from those which 
arise from mere variation of density f- And in some 
examples given by Dr Bancroft, so far are the re- 
fractive arid reflective powers of bodies from being 
in proportion to their density, that they are observed 
to be rather in opposition to it J. 
* On Permanent Colours, p. 7. 
f Ibid- p. 19. : Ibid. p. 23. 
