the Cause of Coloured Concentric Rings. 163 
point would really, as I have before said, be an endless under- 
taking. The only fault, therefore, that may be found with my 
figure, I believe is, that it enters perhaps too particularly into 
this circumstance. It is well known, that similar discussions 
about irises, fringes, or halos, have generally been dispatched 
without giving us the least intimation about the real angular 
course of the rays that produce them. It was indeed, not in- 
cumbent on me to go so far, but where angles and distances 
and intersections fell in my way, that could be determined, I 
was unwilling to pass them by unnoticed, as such considera- 
tions certainly tend to facilitate our conception of the ultimate 
production of the streaks. 
Fourth remark. That such delineations may be used, to 
show 7 in what manner we may conceive intricate optical phe- 
nomena to be produced, I have the authority of eminent 
writers in my favour ; thus Newton, in his first figure of the 
3d book of Optics, assumes four rays on each side, to illustrate 
in what manner we may conceive that a hair can give a pro- 
portionally larger shadow near its body, than at a distance ; 
but no one will affirm, that these four rays can give an idea 
of the actual quantity of light, and the real angles in which it 
falls on the different parts of the paper ; ail which it would 
be necessary to show, in order to prove, that the appearance 
of it on paper, agrees with the hypothesis ; and yet we may 
nevertheless perfectly well conceive the author’s meaning, 
and can make no serious objection to his explanation, on ac- 
count of his having taken but four rays, at lour arbitrary 
distances, moving in four arbitrary angles. 
