Instrument of the same type subsequently mounted at Paris, and in like instruments of intermediate size mounted at other French observatories, the object-glass is placed outside the mirror N, so that both the silvered mirrors are protected from exposure to the outer air.
Observatories.These have in recent years been considerably augmented.
The establishment, in 1671 and 1676 respectively, of the French and English national observatories at once typified and stimulated progress.
Sir David Gill derived a highly satisfactory value of 8.78" for the long-sought constant from the opposition of Mars in 1877, and from combined heliometer observations at five observatories in 1888-1889 of the minor planets Iris, Victoria and Sappho, the apparently definitive value of 8.80" (equivalent distance, 92,874,000 m.).
The opposition of November 1900, though only moderately favourable, could not be neglected; an international photographic campaign was organized at Paris with the aid of 58 observatories; and the voluminous collected data imply, so far as they have been discussed, a parallax for the sun a little greater than 8.8".