22 6 Dr. Herschel's Observations of the Transit of Mercury 
“ That in order to see well with telescopes, it is required that 
“ the temperature of the atmosphere and mirror should be uni- 
“ form, and the air fraught with moisture.” 
This being admitted, we shall find no difficulty in accounting 
for every one of the foregoing observations. 
If an uniform temperature be necessary, a frost after mild 
weather, or a thaw after frost, will derange the performance of 
our mirrors, till either the frost or the mild weather are suffi- 
ciently settled, that the temperature of the mirror may accom- 
modate itself to that of the air. For, till such an uniformity with 
the open air, in the temperature of the mirror, the tube, the 
eye-glasses, and I would almost add the observer, be obtained, 
we cannot expect to see well. See observation 15, 17, 18, 19, 
and 23. 
But, when a frost, though very severe, becomes settled, the 
mirror will soon accommodate itself to the temperature; and 
we shall find our telescopes to act well. See obs. 16, 20, 21, 
22, 24, and 25. 
This explains, with equal facility, why no telescope just 
brought out of a warm room can act properly. See obs. 40 
and 41. 
Nor can we ever expect to make a delicate observation, 
with high magnifying powers, when looking through a door, 
window, or slit in the roof of an observatory ; even a confined 
place, though in the open air, will be detrimental. See obs. 42 
and 43. 
It equally shows, that windy weather in general, which must 
occasion a mixture of airs of different temperatures, cannot be 
favourable to distinct vision. See obs. 31, 32, and 33. 
The same remark will apply to Aurorae JBoreales, when they 
