In the hands of Barnard and others the portrait-lens type of refractor has been of great service in the photography of comets, with their extended tails. This type of instrument has been successfully used also in the discovery of faint comets and asteroids, by means of the slight movement of the comet or asteroid among the surrounding stars, in an exposure of several hours' duration.
An entirely different type of photographic refractor is that which was first used by Rutherford of New York, a great pioneer in this work. About 1870 Rutherford took a fine achromatic visual objective of 12 inches aper ture and 15 feet focal length, and deliberately modified the surfaces of the glasses composing this objective, in such a way as to convert the visual objective into a photographic one. With this telescope Rutherford made a series of fine photographs of the moon and the coarser star clusters. About 15 years later the Henry brothers of the Paris Observatory constructed first an eight-inch, then a 13-inch photographic telescope of a type which proved so successful that it was adopted at the Paris conference of astronomers in 1887 for use in making the International photographic map of the heavens. Observatories of many nations have co-operated in this great work, the telescopes used being of uniform size and power. Each telescope has both a photographic and a visual lens or ob jective, the two objectives being mounted side by side in a double tube. The visual lens enables the astronomer to watch and to intro duce the necessary minute corrections in the motion of the telescope while the photograph is being taken with the other lens. The photo graphic lens or objective does not give a sharp visual image or focus, but does unite in sharp focus all of the rays of light of the blue region of the spectrum, which rays are the ones which are chiefly effective in photography. Similarly the visual lens or objective would not give a sharp photograph, unless special precautions were taken, as described later in this article, but it does unite in one sharp focus all of the rays of light in the yellow region of the spec trum, which rays chiefly affect the human eye. The type of photographic refractor now being described gives sharply defined photographs covering an area of the sky about two degrees in diameter; and by its use the work of chart ing the stars of the entire heavens down to stars of the 13th magnitude has progressed with much success.
The marked success of the type of photo graphic refractor just described has led to the construction of several telescopes of much greater size but of similar type; i.e., with both
photographic and visual objectives. Two of these great instruments, one at Meudon, France, and one at Potsdam, Prussia, are probably the i largest and finest refractors in Europe. They have been used largely in spectroscopic work, and also for direct photography of star-clusters, planetary nebulae, etc.
In the United States, methods very different from the above have been used for adapting for photographic purposes the very large visual refractors of the Lick, Yerkes and other ob servatories. The 36-inch Lick refractor is vided with an additional single lens, 33 inches in diameter, called the photographic corrector, which can be attached to the cell directly in front of the two lenses comprising the visual objective. The resulting combination of three lenses gives a sharp blue or photographic image or focus. This combination was utilized by Burnham, Colton and others in making a very fine series of photographs of the moon. A lunar atlas comprising these photographs was published by the Lick Observatory.
At the Yerkes Observatory a much simpler and more economical method is used for adapt ing the 40-inch visual refractor to photographic purposes. A yellow color-screen or ray-filter is used at the focal plane in contact with a very sensitive or yellow-sensi tive photographic plate. The ray-filter excludes the out-of-focus blue image (which otherwise would destroy the sharpness of the photograph) and allows the sharp and intense yellow image given by the visual objective to pass through and act upon, the yellow-sensitive plate. By this means Ritchey secured a series of very sharp photographs of lunar details, and with the addition of a double-slide plate-carrier for guiding during long exposures, the same ob server secured a series of large-scale photo graphs of the dense globular star-clusters. With the same telescope and with similar methods Barnard secured very fine photographs of the brighter planetary nebulae and of the planets. The use in this way of the yellow ray-filter and yellow-sensitive plates has made all visual refractors available for photographic purposes. In fact several of the largest visual refractors in the world are now being used, by this means, in securing photographs for meas urements of the very greatest refinement and accuracy, including measurements for stellar parallax.