MR. ISAAC ROBERTS ON LONG-EXPOSURE PHOTOGRAPHS. —In the current number of Knowledge, Mr. Isaac Roberts describes a beautiful photograph of Orion, taken by him with an exposure of seven hours thirty-five minutes, the photograph “depicting very probably the maximum of extent and detail that can be shown by aid of photographic methods.” This statement, coming from one so versed in celestial photography, cannot be considered lightly, but must be carefully weighed before judgment be given. The reasons which Dr. Roberts gives for this statement are as follows: (1) The film of the negative is, in consequence of prolonged exposure to the latent sky luminosity, darkened on development to a degree that would obscure faint nebulosity and faint stars. (2) Longer exposures of the plates would not reveal additional details of nebulosity, nor more faint star images. Dr. Roberts goes on to say that, although he has taken all precautions to protect the plates from extraneous light, to photograph only on clear evenings, &c, yet the longer the exposure the darker the film becomes in the development of the images. The sequence, he states further, has been observed for many years on all very sensitive films which have had long exposure, and the results have been practically invariable. An important point, favouring Dr. Roberts’ statement, is that the unexposed margins of the films do not undergo this process of being darkened, but remain perfectly clear. The point raised by him is one well worth consideration in these days of long exposures; and although the evidence he brings together is strong, yet we hope he has not proved his case.