Dr J. Stokes on some Optical Pha:twmena, 417 



The extremities of the remaining two were well defined, the 

 lines connecting them being also parallel to the horizon. 



The distance from the point of contact of 6 c and d e to that of 

 J'g and h i was about 20°. The radius of the circle h c about 10°. 



The air was rather damp, and the barometer also was in- 

 constant, varying with great rapidity. 



I shall now mention the second of these phaenomena. — In 

 the month of July 1825, at 5\ A.M., I had ascended Garry- 

 Castle (one of the highest of the Dublin mountains) to the ele- 

 vation of about 1200 feet above the sea, and 1000 above the 

 bottom of a deep valley situated at its base. A dense cloud had 

 been hanging on the summit the whole morning, and into its 

 gloomy twilight I entered. It was sinking slowly down the 

 mountain into the plain, partly hanging over the valley; in con- 

 sequence of which I soon found the fog decreasing, and the 

 tops of the surrounding mountains dimly revealing themselves. 

 At this time I had my back to the valley ; and my face being 

 towards the sun, (which was now breaking through the mist,) 

 I happened to turn about, when I saw suspended over the 

 former a brilliant semicircular arch of white light, exactly op- 

 posite the sun, and with a radius of about 40°. The breadth 

 of the bow could not be exactly guessed at, because the light 

 was more brilliant at its central part than at the edges, to- 

 wards which it gradually faded away. It could not, however, 

 have exceeded about 5°. 



Its dissolution was caused in about five minutes, by the cloud 

 sinking away into the valley and sailing into the plain. To- 

 wards the conclusion the band of lighi assumed a bkieish hue 

 towards the ground, the sun shining then more strongly. 



The cloud in which I was enveloped seemed to me to be 

 composed of minute bubbles floating about, and apparently 

 about one-fifth the size of a pin's head. The reflection of the 

 sun's rays from the sea had a peculiar yellow colour, and that 

 from the watery particles of the cloud caused him to be lost 

 and confused in a surrounding blaze of light. This was to- 

 wards the end of the pha^nomenon, when the fog was becoming 

 less dense than before. The arch was quite transparent. 

 Some objects about twenty yards off were visible through it. 

 No secondary bow was any where seen. Saussure, I believe, 

 remarked that a cloud which surrounded him on the Alps, 

 was composed of minute globules. 



I shall next describe the experiments on minute lioles. — I 

 punctured a minute hole in a card, and having held it up to 

 tlie light and applied my eye to the hole, 1 placed a pin 

 between them. Imniedialcly 1 saw an enormous and indi- 



A'..S'. Vol. fj. No. fjf). Dec. 1829. 3 H slinct 



