VOL. XXXI.] FHILOSOPHICAL TRANSACTIONS. 541 



An Account of a Rainbow seen on the Ground. Communicated by the Hev, 

 Benj. Langwithy D. D. to Dr. Jurin^ Sec. R. S. N'^ 369, p. 229. 



Sept. 7> 1721, about 9 in the morning, I was riding with some friends over 

 Port Mead near Oxford. The morning had been misty, and the grass was 

 very wet with the dew. We had not been long out, before the air cleared up, 

 and the sun began to shine very bright. We soon after observed a rainbow on 

 the ground, whose colours were almost as lively as those of the common iris, 

 extending for some hundreds of yards; the colours were so strong, that it 

 might have been seen much further had it not been terminated by the bank, 

 and hedge of the jSeld. It continually changed its place as we moved along, 

 as commonly happens in other rainbows. The more remarkable particulars 

 were these. 



1. That its figure was not round but oblong, being I conceive a portion of 

 an hyperbola. 2. That the convex part was turned towards the eye, and the 

 vertex at a small distance before us. 3. That the colours took up less space, 

 and were much more lively, in those parts of the iris that were near us, than in 

 those at a distance. 



These phaenomena may be easily accounted for, by comparing this iris dce, 

 fig. 16, pi. 13, with the common iris keiE formed by drops falling in the air at a 

 small distance from the eye of the spectator h, and touching the ground with 

 the lower part of its arch in e, the vertical point of the iris doe. Produce the 

 cone nkifie, then its intersection with the plane of the horizon will give the 

 figure of the iris dce. Hence it follows, 



1. That as the angle chg happens to be greater, equal to, or less than 90°, 

 the figure will be a hyperbola, parabola, or ellipsis. 2. That as the sun was 

 about 30° high when we viewed the phaenomena, the iris was a hyperbola. 3. 

 That the arches of the same iris, consisting of colours of different refrangi- 

 bility, may also in some cases be different sections of the cone. 4. That since 

 the angle chb' is always given; from the height of the point of view hg, and 

 the sun's altitude sla, the dimensions of these irises are easily determined. 



A Letter from Mr. Anthony Van Leuwenhoeck, F. R. S. on the Pores or Spira- 

 cula of Box-leaves, and on the Down of Peaches and Quinces. Dated Delft, 

 Jan. 15, 1721. Abridged from the Latin. N*^ 369, p. 231. 



It occurred to Mr. L. that the leaves of trees might possibly be provided 

 with spiracula; and having in his area two plants of that sort of box which is 

 commonly called palma ceres [palma cereris] he gathered one of the leaves. 



