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.
Before we come to the discovery itself let us say a little concerning Dr. Janssen’s recent endeavours. Among the six large telescopes which now form a part of the equipment of the new Physical Observatory recently established by the French government at Meudon, in the grounds of the princely Chateau there, is one to which Dr. Janssen has recently almost exclusively confined his attention. It is a photoheliograph giving images of the sun on an enormous scale—compared with which the pictures obtained by the Kew photoheliograph are, so to speak, pigmies, while the perfection of the image and the photographic processes employed are so exquisite, that the finest mottling on the sun’s surface cannot be overlooked by those even who are profoundly ignorant of the interest which attaches to it.
This perfection of size and image have been obtained by Dr. Janssen by combining all that is best in the principles utilised in one direction by Mr. De La Rue, and in the other by Mr. Rutherfurd, to which we have before referred. In the Kew photoheliograph, which has done such noble work in its day that it will be regarded with the utmost veneration in the future, we have first a small object-glass corrected after the manner of photographic lenses, so as to make the so-called actinic and the visual rays coincide, and then the image formed by this lens is enlarged by a secondary magnifier constructed, though perhaps not too accurately, so as to make the actinic and visual rays unite in a second image on a prepared plate. Mr. Rutherfurd’s beautiful photographs of the sun were obtained in a somewhat different manner. In his object-glass, as we have seen, he discarded the visual rays altogether and brought only the blue rays to a focus, but when enlargements were made, an ordinary photographic lens—that is, one in which the blue and yellow rays are made to coincide—was used.
Dr. Janssen uses a secondary magnifier, but with the assistance of M. Pragmowski he has taken care that both it and the object-glass are effective only for those rays which are most strongly photographic. Nor is this all; he has not feared largely to increase the aperture and focal length, so that the total length of the Kew instrument is less than one-third of that in operation in Paris.
The largely-increased aperture which Dr. Janssen has given to his instrument is a point of great importance. In the early days of solar photography the aperture used was small, in order to prevent over-exposure. It was soon found that this small aperture, as was to be expected, produced poor images in consequence of the diffraction effects brought about by it. It then became a question of increasing the aperture while the exposure was reduced, and many forms of instantaneous shutters have been suggested with this end in view. With these, if a spring be used, the narrow slit which flashes across the beam to pay the light out into the plate changes its velocity during its passage as the tension of the spring changes. Of this again Dr. Janssen has not been unmindful, and he has invented a contrivance in which the velocity is constant during the whole length of run of the shutter.