the Cause of Coloured Concentric Rings. 155 
air? The necessity of some cause for the direction of the 
streaks, may be inferred from the experiment which has been 
given in the 35th article. For when a plain slip of glass is 
laid upon a cylindrical curvature, the line of contact, which 
way soever it be turned, will determine the direction of the 
streaks that are to be seen ; but when two plain surfaces 
touch all over equally, no bias of this sort can be given to the 
direction of streaks : the same cause, therefore, which deter- 
mines the direction of the bow, must also determine that of 
its streaks, and this establishes their dependence on critical 
separation. 
Second illustration. In what has been said, the possibility 
that streaks might be formed between two plain surfaces in- 
dependent of critical separation, has been admitted, but this I 
cannot allow. The advocates for the colours by thin plates, 
themselves, must confess, that an uniformly thin plate of air 
between two plain surfaces, ought not to produce streaks, 
which contain a variety of colours, so that the very existence 
of streaks, already proves the action of some principle that 
will produce different colours ; but when the plain side of a 
prism is laid upon a plain surface of glass, in which situation 
it has been proved, by the appearance of the bows, that either 
the reflective or intromissive colour making principle, may be 
made to exert itself at the interior or exterior base of the 
prism, and when in either case, streaks are immediately pro- 
duced, their dependence on the same cause that produces the 
bows, namely, the critical separation cannot be doubted. 
Third illustration. That all the bow-streaks are not only 
dependant on critical separation, but that each collection de- 
pends in particular, on the very principle which forms the 
