He found that in order to bring the actinic portion of the rays to a perfect focus, it was necessary that a given crown-glass lens should be combined with a flint, which will produce a combined focal length of about ⅒ shorter than would be required to satisfy the conditions of achromatism for the eye. This combination was of course absolutely worthless for ordinary visual observation; his new lens when finished was 11¼ inches aperture and a little less than 14 feet focal length. With this he obtained impressions of ninth magnitude stars, and within the area of a square degree in the Prœsepe in Cancer twenty-three stars were photographed in three minutes’ exposure. Castor gave a strong impression in one second, and stars of 2˝ distance showed as double. But even with this method Mr. Rutherfurd was not satisfied. Coming back to the 11¼-inch object-glass which he had used at first, he determined to see whether or not the addition of a meniscus lens outside the front lens would not give him the requisite shortness of the focus and bring the actinic rays absolutely together. By this arrangement he got a telescope which can be used for all purposes of astronomical research, and he has also eclipsed all his former photographic efforts. CHAPTER XXXIII. CELESTIAL PHOTOGRAPHY (CONTINUED)—RECENT RESULTS. Having in the previous chapter dealt with some of the pioneer work, we come finally to consider some of the applications which in the last years have occupied most attention. With regard to the sun, we need scarcely say that Messrs. De La Rue and Stewart have been enabled, by the photographic method, to give us data of a most remarkable character, showing the periodicity of the changes on the sun’s surface, and so establishing their correlation with magnetic and other physical phenomena. These photographic researches, following upon the eye observations of Schwabe, Spörer, Carrington and others, have opened up to us a new field of inquiry in connection with the meteorology of the globe; and it is satisfactory to learn that photoheliographs are now daily at work at Greenwich, Paris, Potsdam, and the Mauritius, and that shortly India will be included in the list. Quite recently, the importance of these permanent records of the solar surface has been demonstrated by Dr. Janssen, the distinguished director of the Physical Observatory at Meudon, in a very remarkable manner. It seems a paradox that discoveries can be made depending on the appearance of the sun’s surface by observations in which the eye applied to the telescope is powerless; but this is the statement made by Dr. Janssen himself, and there is little doubt that he has proved his point.