Meteorological and Astronomical Notices, 381 



vatory, and therefore altogether irremediable by any con- 

 trivances within. It is an evil only to be met by establishing 

 the telescope on the top oF a very lofty mountain, as on some 

 of the inhabited parts of the Himalaya range 16,000 and 

 18,000 feet above the level of the sea, where there would be 

 but half the quantity of atmosphere to battle against, and 

 that of a much purer and more transparent quality, and free 

 from a cause which was very disturbing on some occasions 

 even within the Observatory, — this was atmospheric dust. 



Atmospheric Dust. — On the 11th of September, for instance, 

 the sky being blue and cloudless, and the town almost smoke- 

 less, yet there was an extensive haze, rendering objects even 

 close by grey and indistinct, and producing that peculiar effect 

 in the atmosphere of preventing stars of the first magnitude 

 being seen in the day time in the apparently clear sky with a 

 telescope which, on ordinary days, shows stars of the second 

 and third magnitude, and on remarkable occasions even 

 smaller ones. Here was a cause at once capable of extin- 

 guishing the very faint light of the red prominences ; but 

 there was another mode in which it acted inside the building. 



Though there was no wind, and though there had been no 

 disturbance in the Observatory that day, yet the dome and 

 the telescope were full of dust, only visible in the concen- 

 trated sun- beam coming through the object-glass, but densely 

 and uniformly mixed up with the air, and apparently quite 

 at home in it, and showing no tendency to any intention 

 of subsiding. Though the particles were excessively small, 

 yet they were so very numerous and reflective, and they 

 spread the light of the sun about to such an extent in its 

 passage from the telescope to the black box, that the re- 

 quisite degree of darkness could not be maintained in the 

 room, besides producing a disturbing effect on the definition 

 of the image thrown on the screen. It is probably when dust 

 is mixed with the air, that the superiority of a Cassegrainian 

 telescope over a Gregorian, or one where the rays are not 

 brought to a focus before coming to the eye, is most apparent, 

 or perhaps only apparent at any time, by reason of the air never 

 being altogether free from some of these floating particles. 



And what are these particles, and what purpose do they 



