S′O, and is thus reflected in the direction OG′, then by the law of reflection, S′O and G′O make
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Fig. 13.
equal angles with the surface of the quicksilver. But as the quicksilver is perfectly fluid, its surface is exactly horizontal. So that G′O and S′O make equal angles with the horizon; and therefore F′G′O points as much below the horizon as OS′ or FGS points above it. The observer therefore looks at the star by reflection in the quicksilver; he takes the reading of the microscopes; he then turns the instrument so as to see the star by direct vision in the telescope, and then he takes the reading of the microscopes; then by taking the mean between the reading of the circle corresponding to these two observations, it is certain that we have got the reading corresponding to the horizontal position of the telescope. That gives us a starting-point; and having got that, whenever we
