220 Dr. Herschel’s Observations of the Transit of Mercury 
(14.) April 6, 1783. A very thick fog settles upon all my 
glasses ; but the specula, even the 20-feet, which has so large a 
surface, remain untouched. I see perfectly well. 
Frost. 
(15.) Nov. 15, 1780; 5 o’clock in the morning. An excel- 
lent speculum, No, 2, will not act properly; the frosty morning 
probably occasions an alteration in its figure. Another speculum, 
No. 1, acts but indifferently, though I have known it to shew 
very well formerly in a very hard frost : for instance, November 
23, 1779, I saw with the same mirror, and a power of 460, the 
vacancy between the two stars of the double star Castor, without 
the least aberration. 
(16.) Oct. 22, 1781. Frost seems to be no hindrance to 
perfect vision. The tube of my 7* feet telescope is covered with 
ice ; yet I see very well. 
(17.) Nov. 19, 1781. It freezes very hard, and the stars, 
even those which are 50° high, are very tremulous. I suspect 
their apparent diameters to be diminished ; and, if I recollect 
right, this is not the first time that such a suspicion has occurred 
to me. 
(38.) Jan. 10, 1782. My telescope would not act well, even 
at an altitude of 70 or 80 degrees. There is a strong frost. 
(19.) Jan. 31, 1782. I cannot see with a power of 460, the 
stars seem to dance so unaccountably, and yet the air is per- 
fectly calm : even at 60 or 70 degrees of altitude, vision is 
impaired. 
(20.) Feb. 9, 1782. That frost is no hindrance to seeing 
well is evident; for, not only my breath freezes upon the side 
