22 Mr. Vince’s Observations upon 
a!" E, and b" E, b'" E, and these lines must respectively be the 
directions of the rays entering the eye from a and b, in order 
to produce the images a " b" and a'" b"'\ hence, these lines must 
be tangents at E, to the curves which are described by the rays 
of light ; let therefore a n E, a m E, b v E, b w E, be the curves 
described. We have therefore to assign a cause which may 
bring rays passing above the rays ar E, bs E, to the eye 
at E. Now, if there were no variation of the refractive power 
of the air, a ray of light passing through it would describe a 
straight line ; therefore, the curvature of a ray of light pass- 
ing through the atmosphere, depends upon the variation of 
the refractive power of the air. If, therefore, we suppose the 
air lying above ar E, to vary quicker in its refractive power 
than the air through which arE passes, the curvature of a ray 
proceeding above that of a r E, will be greater than the curva- 
ture of ar E ; and upon this principle we may conceive that a 
ray may describe the curve an E : and, in like manner, if a 
quicker variation of refractive power should take place above 
the curve an E, than in that curve, a third ray may describe the 
curve am E. The same may be said for the rays bv E, bw E, 
diverging from b. The alterations of the refractive power may 
arise, partly from the variations of its density, and partly from 
the variations of its moisture; and the passage of the rays 
through the boundary of the fog may there suffer a very con- 
siderable refraction ; for, from the motion of the fog, and that 
of the images abovementioned, I have no doubt that the fog 
was a very considerable agent in producing the phsenomena. 
When all the causes co-operate, I can easily conceive that they 
may produce the effects which I have described. If the cause 
should not operate in the tract of air through which the curves 
