228 Dr. Herschel’s Observations of the Transit of Mercury 
It is however possible, that fogs and haziness may increase 
to such a degree as, at last, to take away, by their interposition, 
all tbe light which comes from celestial objects ; in which case, 
they must of course put an end to observation ; but they will 
nevertheless be accompanied with distinct vision to the very last. 
See obs. 44, 45, and 4 6 . 
We have now only the four last observations to account for. 
They relate to the change of the focal length of mirrors in 
solar observations, and its attendant derangement of the foci of 
the different parts of the reflecting surface ; and, as simplicity 
is one of the marks of the truth of a principle, I believe we need 
not have recourse to any other cause than the change of tem- 
perature produced by the action of the solar rays that occasion 
heat; which will be quite sufficient to explain all the pheno- 
mena. But, in order to show this in its proper light, I shall 
relate the following experiments. 
1st Experiment. 
I placed a glass mirror, of 7-feet focal length, in the tube be- 
longing to the telescope ; and, having laid it open at the back, 
I prepared a stand, on which the iron used in my experiments 
on the terrestrial Rays that occasion Heat (see Phil. Trans, 
for 1800, Plate XVI. Fig. 1 ) might be placed, so as to heat the 
mirror from behind, while I kept a certain object in the field of 
view of the telescope. Having measured the focal length, and 
also examined the figure of- the mirror, which was parabolical, 
the heated iron was applied so as to be about 2^ inches from 
the back of the glass mirror. The consequence of this was, that 
a total confusion in all the foci took place, so that the letters on 
a printed card in view, which before had been extremely distinct. 
