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not having been invented, he could not use
a telescopic-sight as we now do in gunnery.
This not only removes the difficulty of fo-
cussing, but makes the minimum visible an-
gle smaller. Helmholtz has defined the min-
imum angle measurable with the naked eye
as being one minute of arc. In view of this
it is simply marvellous that, when the po-
sitions of Tycho’s standard stars are com-
170
