the Cause of Coloured Concentric Rings. 161 
of glass, supposed to be plain, may be shown, which, when 
put together slightly inclined, as the experiment requires, will 
produce streaks near the line of contact ; but, if this should 
be the case, my 35th article accounts so well for the appear- 
ance of such streaks, that it would not be philosophical to 
ascribe them to plain surfaces, when it has been shown, that 
cylindrical curves of any figure, will invariably produce them ; 
for which reason, I should think myself justified in conclud- 
ing, that one or other of the plates, which were supposed to 
be plain, had a cylindrical termination ; the figure of which 
might be circular, elliptical, parabolical, hyperbolical, or 
indeed of any other variety of cylindrical curvature. 
LVIII. Illustrating Remarks on the Intention of the i/gth Figure, 
explained in the 48 th Article of my Paper. 
The great difficulty of representing rays of light, which 
are compressed beyond all conception, is such, that even a 
figure one thousand times magnified, which gives a delinea- 
tion of them, is hardly less inadequate to give a tolerable idea 
of what is to be expressed, than if it had not been at all in- 
creased in its dimensions. This being the case, it might be 
expected that some objections would arise, such as that in my 
figure constructed for explanation of the streaks the vacancies are 
observed to correspond with , and to depend upon the intervals be- 
tween the rays 1, 2, 3, 4, &c. originally assumed as separated by 
blank intervals . There may appear to be some plausibility in 
this objection, but still if some notion of this kind should be 
entertained, it may be shown, not only that such a remark 
would not be quite correct, but also, that the supposed force 
of it, is founded on a misconception of the figure. 
