1319.] Dr. Watt on the Formation of the Rainbow, 131 
RAIN. 
1818, Inches, 1818. Theles: | 1818. Inches. 
July 7 0°331 Aug. 3 0-072 Sept. 0-288 
9 0:036 5 0-060 ‘ 0-241 
I 0-024 9 0-030 0-060 
20 0-213 18 0045 0-609 
22 0-041 24 0:003 0-017 
24 0-048 27 0-051 0-066 
25 0-309 29 0-015 0-030 
25 6-138 31 0-219 14, 0-042 
29) 0°141 ae Pe 15 0-144 
31] 0-216 0°495 16 0-048 
4 is} 0-063 
1-497 19} 0-243 
2) 0°325 
22} 0-198 
| 23| 0-729 
24 0-270 
26, 0-126 
27 0-441 
28}  0°390 
| 30| 0°378 
4-108 
ArticLe IX. 
On the Formation of the Rainbow. By Robert Watt, M.D. 
(To Professor Thomson.) 
DEAR SIR, ~*" Glasgow, Nov. 4, 1818. 
Azour fifteen years ago I was engaged in a variety of 
researches respecting the nature of light and heat, which led me 
to pay particular attention to what have been called the primitive 
colours, and more especially as they appear in the rainbow. For 
a time I was satisfied with the Newtonian theory, and of course 
all my views were bounded by that hypothesis. The first thing 
which tended to stagger my belief in it was that I could, in few 
instances, satisfy myself that there were really drops of rain 
falling at the place where the rainbow appeared. A rainbow 
was often seen in a direction in which for many miles not a drop 
of rain had fallen. It occurred to me, therefore, that to make 
the Newtonian theory complete, the existence of drops of rain 
should first of all have been demonstrated. 
Without troubling you with my doubts and difficulties, I shall 
shortly state what gave me an entire new view of the subject. 
One day, I think in 1805, while I was observing a very vivid 
rainbow, I happened to turn my eyes towards the sun, and 
observed that he was passing along the lower edge of a sort of 
12 
