318 On a Lunar Iris, or Itainboli) hi) Moonlight. 



over Portsdown-hill, the iris as yet incomplete : at 55 minutes 

 past six, the moon shinuig in a clear siiace, with an altitude of 

 only Ti*^, a distance of 29° westward of the meridian, and 

 the rain moderately descending, our gratification was com- 

 pleted, at the expense of a wetting, by the appearance of a 

 perfect lunar iris, of a silvery colour in a black passing nim- 

 bus or rain-fraught cloud to the N.E. No prismatic colours 

 were distinguishable in any part of the iris, the moon not 

 having come to her first quarter; consequently her light 

 seemed too faint to })roduce a variety of colours in the bow. 



The extent of the iris along the plane of the earth's surface 

 was 81° 34.'; that is, within 3° of its greatest extent, on the sup- 

 position of an observer standing on a plain ground, with the 

 moon in the horizon : and the altitude of its apex above the 

 horizon was upwards of 27°. By comparisons of this mea^ 

 surement with solar rainbows, when the sun has had a similar 

 altitude, we conclude that there is no perceptible difference in 

 the extent of the solar and lunar iris. 



On the 19th December 1820, vre observed a faint lunar iris 

 to the westward, in a thick fog, from 7 till 8 P.M. ; but the 

 altitude of the moon being upwards of 30°, the apex of the 

 h-is was not more than 10° or 11° above the horizon. 



On the 15th October 1820, we observed a perfect lunar iris 

 to the N.E., with part of an exterior bow above it, during a 

 shower at 9 P.M., m which some faint prismatic colours were 

 discovered, the moon at that time being ahnost in the middle 

 of her second quarter. 



On the 25th of August 1817, we also observed a perfect 

 lunar iris to the N.W. from 30 to 40 minutes past 8 P.M. on 

 a large but slowly passing nimbus, the moon being to the S.E. 

 and nearly full : prismatic colours were distinctly traced in this 

 iris ; but they certainly bore no near comparison with those of 

 the solar rainbow that appeared the following afternoon. 



In all these appearances of the limar iris, the weather at the 

 respective times was misettled and stormy for several days to- 

 gether : the jihasnomenon, however, is not a prognostic, but 

 rather the effect of a series of sudden storms. 



From these ren^arks we may conclude, that the best time to 

 look out for such a rare phaenomenon is in stormy weather, 

 when the nimbi pass in quick succession over us, and when 

 the moon has a low altitude and is at least five or six days old 

 (12 or 14 or 16 would be better): but the nearer she is to the 

 horizon at either rising or setting, the more beautiful and ex- 

 tensive will the silvery arc appear with its delicate prismatic 

 colours in the front of the rain-cloud, which must come from 

 or near the- quarter in which the moon appears. 



