1. Introduction. Atlas Photographique de la Lune.
The current and past conditions of the lunar surface, which more or less exhibit the various manifestations of nature's forces, form a chapter of physical astronomy that has resulted in much research and has excited considerable and sustained public interest. Early in the invention of the telescope, observers made use of maps to represent all the details that could been identified with certainty. Many publications followed, each building on the most recent advances in optical instruments. But the outcome of all these documents, while highlighting many instructive facts, has disappointed their early hopes. All the important features of the lunar relief are now well known, those that the complexity of the Moon's motion does not hide from our view. But while the Sun, four hundred times further away, offers the observer a scene of constant activity, the Moon appears frozen in an eternal immobility. Changes that early observers thought present had to be put down to errors of measurement or drawing. If some are real as is likely as a result of the patient investigations of Madler and Schmidt, who highlighted details too delicate and too doubtful to be interpreted, what can be said in each of these cases of the physical forces which intervened and what was their mode of action.
Having learned from experience, selenographers must now adopt a more modest goal, but surely a more feasible one. All bodies undergo successive transformations. The surface of the moon can be no exception and aggregate changes that occur over centuries can not escape forever from the optical power of our instruments. They should therefore be given time to work for us, and we must especially prepare for the future by obtaining the current state of our satellite's representation in as true and accurate a form as is possible. In this respect, the superiority of photography over all methods of drawing no longer needs to be demonstrated. Even if the sensitive plate did not succeed in collecting some minute details visible in the telescope, it imparts to its images a character of indisputable authenticity, to which all the dexterity of the draftsman can not supply. This will avoid the uncertainties which have always prevented definitive conclusions from being drawn from the old works.
From another point of view, the photographic method imposes itself with equal authority. In spite of the great skill and the immense labour spent to establish the existing maps of the Moon, it is certain that the correspondence between the innumerable drawings on which they were based could not always be carried out with precision. Continuous changes in appearance due to the effects of solar illumination, libration, and refraction make this fusion very difficult to carry out, even assuming that each of the partial documents is irreproachable. A glance at the photographs shows, in fact, that the main lineaments of the great formations are faithfully reproduced on the best maps. The almost instantaneous recording of a considerable fraction of the lunar surface constitutes in this respect an inestimable advance, and no other way is offered to arrive at a truly homogeneous representation, comparable to itself in all its parts and relative to a clearly defined era. This is so true that the lack of a general and intuitive view is even evident in the best geographical atlases obtained from the juxtaposition of separate studies. The relief of certain restricted portions of our globe is known at present, it is true, with a meticulousness which selenographic studies do not imply; but it would still be impossible to give a picture of the vastness of the Earth as eloquent and as faithful in the distribution of shades, as those given to us by the lunar plates.
2. Popular Astronomy account of Maurice Loewy and Pierre Puiseux's work.
This is a work upon which they have been engaged for some years, and it will be of great value to astronomers. The plates are being reproduced on an enlarged scale by photogravure from negatives taken with the large equatorial, which is the most powerful instrument of the Paris Observatory. The photographic objective of the great equatorial was made by Messrs. Henry, and is of 60 cm aperture and 18 metres focus. Usually the lens is stopped down to 54 cm aperture. The correction of the lens is for the chemical rays, and the adjustment of the focus is repeated each month photographically by trial and error.
The image of the Moon in the original negatives varies between 160 and 180 mm, according to the distance of the satellite from the Earth. The exposures with Lumiere, or Helios, extra rapid plates vary between 1/2 and 4 sec, according to the phase, and the clearness of the atmosphere. The lens remains stationary, but the plate is moved by a Foucault clock and cylindrical gearing. As the movement of the Moon is variable, the problem is a complicated one, but 3,600 different rates of speed may be obtained by the mechanism, and it is therefore easy to make suitable adjustments. The shutter is a sheet of aluminium placed immediately in front of the plate. It slides in grooves, and is controlled by hand, by means of a rod and crank, so that the brightest parts of the Moon's surface receive least exposure. A tubular sky-shade 1.60 m. in length is placed on the front of the lens when photographs are made during twilight. The plates are developed with amidol and anhydrous sulphite of soda to preserve the greatest transparency in the deposit. The enlargements are made by electric arc light and the lens used is a Lerebours and Secretan single achromatic meniscus of 310 mm focus, with convex surface to the light, stopped down to 10 mm. This gives perfect definition of the grain of the negative over the entire surface of the enlargement 80cm by 80cm. Various other lenses have been tried, but none have given such satisfactory results.
Different methods of focussing have been tried, but the best criterion of definition has been found in the grain of the plate as seen in the image upon a white screen. A slight displacement of the lens removes the grain and gives a more agreeable visual image, but for exactness the grain is retained. To obtain equality of exposure and full detail in the shadows, the light is passed through a screen with a slit a few millimetres wide before it reaches the condenser. This screen is kept in motion, and can be used locally. The scale of enlargement varies between 10 and 16 times. The plates are uniform in size, 181/2 inches by 221/4 inches, printed on heavy cardboard of 311/2 inches by 231/2 inches in size backed with dextrine and red ochre.
These pictures also show a remarkable equality of exposure, with full detail in the shadows. Anyone acquainted with photography readily realizes that this is a difficult thing to do. In order to accomplish this the light was passed through a screen with a slit a few millimetres wide, before it reached the condenser. This screen was kept in motion, and it was so arranged that it could be used locally, and thus secure the proper comparative exposure for parts adjacent to one another under different degrees of brightness. In this way a single plate may be made to do better work in reproductions than is possible without the use of some such effective device.