The light of the moon is so feeble in blue rays that a long exposure is necessary for a large image, and during the exposure all the errors in the rate of the clock are magnified. We need not enlarge on the extreme importance of what Mr. Rutherfurd has been doing in photographing star clusters and star groups. It is doubly important to astronomy, and starts a new mode of using the equatorial and the clock; in fact, it gives us a method by which observations may be photographically made of the proper motion of stars, and even the parallax of stars may be thus determined independently of any errors of observers. Mr. Rutherfurd shows that the places of stars can be measured by a micrometer on a plate in the same way as by ordinary observation; hence photography can be made use of in the measurement of position and distance of double stars. As an instance of the extreme beauty of the photographs of stars produced by a proper instrument, it may be stated that with the full aperture of the 11¼-inch object-glass corrected only for the ordinary rays, Mr. Rutherfurd found that he required an exposure of more than ten seconds to get an image of the bright star Castor; but now, instead of requiring ten seconds, he can get a better image in one. The reason of this is, that, with the object-glass corrected only for the visual rays, the chemical ones are spread over a certain small area instead of coming to a point, and so, of course, the intensity is reduced; but when the chemical rays all come to one point the intensity is greater, since the image of the star is smaller and the action more intense. Let us follow Mr. Rutherfurd a little in his actual work. First, a wet plate is exposed for four minutes. This gives stars down to the tenth magnitude. But there may be points on the plate which are not stars, hence a second impression is taken on the same plate after it has been slightly moved. All points now doubled are true stars. Now for measures of arc. Another photograph is taken, and the driving clock is stopped; the now moving stars down to the fourth magnitude are bright enough to leave a continuous line, the length of this in a very accurately known interval, say two minutes, enables the arc to be calculated.