'Singular Apjyearajice of a Rainbow, 449 



" On the 6tli of August, 1698, Dr. Halley, when walking 

 on the walls of Chester, observed a remarkable rainbow. A b 

 is the primary bow ; c d, the secondary ; and f' e' e f, the 

 new bow intersecting the secondary bow c d, and dividing 

 it nearly into three equal parts. Dr. Halley observed the 

 points e' e to rise, and the arch [of the new bow] e' e gradually 

 to contract, till at last the two e' e [of the secondary bow], and 

 e' e [of the new bow], coincided ; so that the secondary iris, 

 for a great space, lost its colours, and appeared like a white 

 arch at the top. The new bow (f' e' e f) had its colours in 

 the same order as the primary one (ab), and, consequently, 

 the reverse of the secondary bow ; and, on this account, the 

 two opposite spectra at e' and e counteracted each other, and 

 produced whiteness. The sun at this time shone on the river 

 Dee, which was unruffled; and Dr. Halley found that the 

 bow f' e' e f was only that part of the circle that would have 

 been under the castle, bent upwards by reflection from the 

 river. 



" A third rainbow, seen between the two common ones, 

 and not concentric with them, is described in Rozier's Jour- 

 nal, and is doubtless the same phenomenon as that observed 

 by Dr. Halley." 



Now, though it may be very presumptuous in an anony- 

 mous individual like myself, to object to an explanation 

 sanctioned by the names of Halley and Brewster, yet I am 

 obliged to say that it is as unsatisfactory to me as it seems to 

 be to E. G. ; and that I, therefore, consider the problem as 

 yet unsolved. I cannot imagine the sort of reflection sup- 

 posed by it possible ; except on such an inadmissible principle 

 as that the rainbow is the base of a hollow cone of prismatic 

 light proceeding from the sun, and rendered visible by being 

 received upon falling rain, or the clouds, or the earth, as a 

 screen. Still, however, as in the instances mentioned, the 

 phenomenon seems to have been observed in the neighbour- 

 hood of an extent of water, it may somehow depend on 

 reflection ; but, as it would be almost vain to attempt a solu- 

 tion without being perfectly acquainted with such circum- 

 stances as the position of the observer, and the relative 

 situations of the bow and the water, as well as the state of 

 the latter, it would be useful if E. G. would oblige your 

 readers with such particulars of the instances witnessed by 

 him as he can now recollect, in reference to these points. 



E. G.'s question ought not to be lost sight of, " What 



becomes of the received opinion that the eye of the observer 



must be in the apex of the cone of which the bow is the 



base?" (III. 5^5,) How is that opinion to be reconciled 



Vol. VII. — No. 41. gg 



