The actual gnomon is not that of 1729. The copper plate, which had been pierced to form the eyepiece, has disappeared. From the pavement of the Cassini room, the visitor can simply note the presence, high up, of a hole letting in very bright light; this is actually a biconvex lens of 85 mm in diameter, now acting as a gnomon. We can't precisely date the replacement of the plate by a lens.
The use of a lens as gnomon is not normal. In the present day its use at the Observatory can be justified as it serves simply to make the image of the Sun brighter by focusing its light as sharply as possible in this large room which is open to the already widely diffused daylight through its high windows (almost seven meters high). Formerly, the observations were made in total darkness, created by thick, heavy blackout curtains.
However, away from the narrow focus, the image becomes increasingly blurred, it is the other side of the lens: the sunspot appears very fuzzy around the winter solstice and the equinoxes and then, as we approach the summer solstice, the contours of the solar ellipse become more distinct. We were able to determine approximately two periods of the year when the sun's image comes to focus fully on the ground. This is on May 5 and 6 August. This gives the lens a focal length of 11.8 m. Charles Le Monnier relates in his History of Heaven (1741) an observation made July 4, 1681 on the temporary meridian line of the Paris Observatory: this was an observation for "determining the summer solstice by the Roemer method, that is, by observing the location of the solar image transmitted by a glass lens on the meridian line. The glass that was used for this purpose had a focal length of 34 feet , and, at noon, we found on the gnomon that the image returned to precisely the same place where it had occurred on June 7". The focal Length of 11.8 m of the lens is 36 feet, which is very similar to that used by Cassini and Roemer, and which thus suggests that these two lenses were in fact one and the same.
