416 D''. -T' Stokes on some Optical Phenomena. 



Table co?icluded. 



F. W. Bessel. 



LXIV. On some Optical Phcenometm. By Dr. J. Stokes*. 



AS facts are the groundwork of science, and the commu- 

 nication of facts to those that love it the only means of its 

 advancement, I shall give a detail of several optical pheno- 

 mena of very rare occurrence, and also of some experiments 

 on the transmission of light through very small holes. 



On the 25th of August in this year, between 7 and 8 A.M., 

 I witnessed the following: 



There was a westerly wind, and the sky was thickly strewed 

 with light fleecy clouds, so small as to resemble flocculi, and 

 so numerous as to form a continuous layer. These were 

 moving with great rapidity towards the sun. In fig. 1, let 

 the point a represent the zenith, and a a" a line passing from 

 thence to the sun. This line appeared to me to pass through 

 the points of contact that four arches be, d c,fg, h i made with 

 one another. They were rather faint, but exhibited prismatic 

 colours ; those of b c being the most perfect, those of h i the 

 least \fg cannot be properly called an arch. Its form possessed 

 rather a striking analogy to that of a well-known curve ; h c ap- 

 peared to be an arc of 60° to a circle whose radius was in the 

 zenith a, and its chord I think parallel to the horizon ; its two 

 extremities b and c were well defined : but those of d e v.'ere 

 not so ; one of them, d, extending indefinitely on that side. 



Communicated by the Author. 



Tht 



