196 Dr. Forster on the Variation of the reflective, 



by some dispersive power imparted to the circumjacent atmo- 

 sphere by the first tailing of the dew. 



The state of the atmosphere which shows the red colour for 

 any length of time is a forerunner of high wind. 



There is an obscurity of a denser kind which often causes 

 an apparent dilatation of the disks of the celestial bodies, which 

 it elevates without colouring them. The moon for instance 

 looks pale, expanded and confused. This state is a prognostic 

 of rain, and is contrasted to the former as well as to the clear- 

 ness of the serene sky which follows an ordinary fall of dew, 

 in a well known proverb expressive of the prognosticative co- 

 lour of the moon on each several occasion : — 



" Pallida luna pluit, rubicunda flat, alba serenat." 



Another state of the cirrostratus, when diffused, seems to 

 possess dispersive properties of a very peculiar nature, re- 

 fracting certain coloured rays at an angle of about 5°; and 

 others at an angle of about 23°, so as to cause two concentric 

 halos or rings of colours ; the inner one being about 10°, and the 

 outer one about 4-6° in diameter. Sometimes single rings occur 

 which are colourless ; but as all these phaenomena do not pro- 

 perly belong to our subject, and are otherwhere treated of, I 

 shall not dwell longer on them. 



All the above circumstances show that the changes in the 

 qualities of the diffused vapour in the air must cause a great 

 variety in the atmospherical refraction. They also explain how 

 the atmosphere about different places may have a different 

 mean refractive power, according to the local prevalence of 

 the above diversified vapours which hang in it. For I am not 

 certain whether, independent of temperature and pressure, the 

 pure atmospheric air differs in its refractive power all over the 

 world : I believe the difference to arise from the vapours always 

 more or less suspended therein and generally electrified. 



Two more facts confirming my hypothesis must now be 

 cursorily mentioned. The east wind produces changes in 

 the appearance of the spectra of the celestial bodies, even 

 in the field of ordinary telescopes : the spectrum seems to vi- 

 brate or dance about in the field of the glass in a way that 

 defeats many very delicate observations. Had not this re- 

 mark been confirmed by other people, I should have ascribed 

 it to the nervousness of the observer, the east wind being 

 very liable to produce nervous disorders of the eyes and other 

 parts. I have not observed the precise effect of the east 

 wind on the larger prismatic spectra, as far as respects their 

 colours ; but the v hole spectrum seems often on such occasions 

 to move, and to fluctuate more than ordinarily. 



Astronomers, ever since the time of Newton, have known 



that 



