used, account must be taken of the gradual diminution in the length of a
degree of right ascension as we leave the equator.

Even simpler methods carefully applied may serve to give a view of
Mercury. To show this, I may describe how I obtained my first view of
this planet. On June 1st, 1863, I noticed, that at five minutes past
seven the sun, as seen from my study window, appeared from behind the
gable-end of Mr. St. Aubyn's house at Stoke, Devon. I estimated the
effect of Mercury's northerly declination (different of course for a
vertical wall, than for the cross-rod in fig. 8, which, in fact, agrees
with a declination-circle), and found that he would pass out opposite a
particular point of the wall a certain time after the sun. I then turned
the telescope towards that point, and focussed for distinct vision of
distant objects, so that the outline of the house was seen out of focus.
As the calculated time of apparition approached, I moved the telescope
up and down so that the field swept the neighbourhood of the estimated
point of apparition. I need hardly say that Mercury did not appear
exactly at the assigned point, nor did I see him make his first
appearance; but I picked him up so soon after emergence that the outline
of the house was in the field of view with him. He appeared as a
half-disc. I followed him with the telescope until the sun had set, and
soon after I was able to see him very distinctly with the naked eye. He
shone with a peculiar brilliance on the still bright sky; but although
perfectly distinct to the view when his place was indicated, he escaped
detection by the undirected eye.[12]

Mercury does not present any features of great interest in ordinary
telescopes; though he usually appears better defined than Venus, at
least as the latter is seen on a dark sky. The phases are pleasingly
seen (as shown in Plate 6) with a telescope of moderate power. For their