If a reason had to be given in addition to the obvious advantage of this method of publication by printing the photographs with permanent ink it would be afforded by the fact that the records obtained by photography are peculiarly liable to be lost by accidental breakage of the glass negatives. Besides this there is the certainty that after the lapse of a limited number of years the gelatine films will become discoloured; the images will fade, and the faint stars and the faint nebulosities will entirely disappear from view.

I have had within my own experience proofs that the faint stars fade from the films, and will give the following examples:–On the 15th February, 1886, a photograph was taken of the region of the sky with the co-ordinates R.A. 9h. 40m. Dec. North 72°·0 at the centre of the plate; exposure 15m.; area of the plate four square degrees.

Shortly after the photograph was taken I counted 403 star-images on the negative; and on 29th May, 1895, I again counted the stars on the same negative, and found only 272. Therefore, stars to the number of 131 had entirely disappeared from the film in the course of nine and a-quarter years.

Another photograph of identically the same region was taken with an exposure of fifteen minutes on the 22nd March, 1886, and soon after that date I counted 364 stars upon the negative.