Discovery of the Ninth Satellite of Jupiter.

So far as the program has gone, therefore, all the evidence points to a great distance for these objects. As the spirals are undoubtedly in revolution-any other explanation of the spiral form seems impossible-r the failure to find any evidences of rotation would indicate that they must be of enormous actual size, and at enormous distances from us. At a distance of one-thousand light-years, supposing mass conditions to be the same as in our solar system, a nebular structure one minute of arc in apparent distance from its nucleus, or 60 times the distance of Neptune from the Sun, would have a yearly angular movement of only 53", or a maximum yearly displacement in a given component with reference to the nucleus of only 0.015". A much greater time interval will probably be necessary before nebular rotations can be definitely established by measures of position, unless a nebula abnormally close to us be found. Wolf and Slipher have reported the observation of rotation in the spiral nebulae Messier 81 and N. G. C. 4594, respectively, by means of the velocity-displacements of lines in the spectra of these bodies. The spectrographic method, here as elsewhere, is independent of the distance or absolute size of the object.