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these small objects, on a sufficiently large scale for the most refined measurement. One great purpose of this work is to detect changes of form, that is, evidences of development, in nebulae. It is plain that the larger the scale of the photographs and the finer the detail shown, the greater is the chance of detecting changes of form within a reasonable time.

Second: By the addition of a convex magnifying mirror, twenty-eight inches in diameter, used in conjunction with the eight-foot mirror, the focal length of the eight-foot reflector could be increased. to four by forty-eight feet-192 feet, or even to six by forty-eight feet-288 feet. Used in this way this telescope would be suitable for photographing the kinds of objects which I photographed with the forty-inch refractor, with the yellow color screen, namely, bright and small objects, such as the globular star clusters, the planetary nebulae, the moon, and the planets; but it would be incomparably more efficient for this work than the forty-inch, for the following reasons: (1) the scale would be greater in the proportion of 288 to 63 (sixty-three feet is the focal length of the forty-inch); (2) the reflector would give greater speed on account of the small loss of light.

I feel certain these photographs would prove as revolutionary as those of the faint nebulae obtained at the primary focus. To illustrate the image of the moon on the original negatives, as photographed with the forty-inch refractor at the Yerkes Observatory, is seven inches in diameter; the image given with the eight-foot reflector, with equivalent focal length of 288 feet, would be thirtytwo and one-fifth inches in diameter. The same ratio of increase holds for the planets, star clusters, etc.

Third: In the photography of the spectra of stars and nebulae, the eightfoot reflector would enormously surpass all existing instruments. In this work it is simply a question of collecting the greatest possible amount of light into the star image. As the eight-foot would give seven times as bright a star image as the Crossley reflector, an immense number of stars, now inaccessible, would be brought within range. As soon as their spectra can be photographed, their chemical com

position and the approximate temperature and pressure in their atmosphere can be determined, as well as their motions toward or away from the earth. The light of some of the bright stars could also be analyzed as completely as that of the sun has been. More important than this, however, is the fact that it would at once become possible to trace out the evolution of stars, and their development from nebulae, with far greater certainty than at present.

Fourth: The only measurements we have of the heat of the stars were made with the two-foot mirror at the Yerkes Observatory. They showed that Arcturus gives us about as much heat as a candle at a distance of six miles. The eight-foot reflector would give sixteen times as much heat as the two-foot, and permit the accurate measurement of a great number of stars, whereas only two stars could be measured in the earlier work.

I hope the above will give you the information which you desire. İf you wish for further details, however, I shall be most happy to furnish them. Very sincerely yours,

G. W. RITCHEY, Superintendent of Instrument Construction.

A brief account of the various processes which will be employed in the manufacture of a hundred-inch mirror will enable one to form some idea of the obstacles which must be overcome, and to appreciate the infinite pains and study essential to the successful completion of the work. To construct a lens weighing four and one-half times more than any in existence it will be necessary to make many changes in the accessories of the glass works and instrument shops.

At the ancient glass works of St. Gobain a specially devised crucible of fireproof clay will be heated gradually for many days and placed in a melting-oven until white-hot. The materials to compose the glass will be slowly introduced through an opening in the top of the oven, the operation consuming possibly forty-eight hours. In case there is no accident to the melting-pot or oven, which are frequently cracked by the tremendous heat, the impurities are skimmed off as they rise to the surface, and the whole.

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Showing nebulosity and tail. Photographed during one of its periodic returns. The nucleus, which is the densest and most luminous part forming the true body of the comet, is hidden by the nebulosity.

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DOUBLE STAR CLUSTER IN PERSEUS. Viewed through a telescope, this is one of the most fascinating of the sections of the heavens. Myriads of suns are here massed.

mixture is thoroughly stirred for several hours with a long clay cylinder. When the liquid mass reaches the correct stage of fluidity and assumes the proper color the great oven is opened at the rear and a two-wheeled truck, with very long handles, is brought close up to it; projecting arms of the truck engage rings on the crucible and the glowing pot is slowly lifted and drawn out over a great circular iron mold. Oftentimes this operation is hindered by the overflow of melted glass, which is likely to cement the pot to the oven floor, whence efforts to dislodge it frequently result in the breakage of the brittle crucible. An iron band is placed about the pot, to reinforce it, and by means of grappling-irons the crucible is tipped until its contents flows into the mold; this is a moment of tremendous interest and importance. The mold is then covered with an iron plate and removed to the cooling-oven, which is already heated to the proper temperature. Here it is walled up and left from six to eight weeks, the temperature being gradually decreased until the glass is cold. The four and a half ton disk of glass will then be removed for rough grinding and polishing, merely preparatory to an examination for possible defects. A few bubbles do not matter, as they have almost no effect, but the existence of cracks or veins will mean that the whole process must be repeated. Sometimes a glass of only thirty-six inch diameter has required ten or a dozen meltings; these difficulties will be multiplied in making the hundred-inch lens. A final treatment in the furnace-house for an

nealing will be necessary, when the lens

will be heated even more carefully than before and allowed to cool gradually for many weeks. It should then be ready for shipment to America.

On the arrival of the lens at the instrument shops in Pasadena the delicate work of making the finished optical mirror will be undertaken by Prof. G. W.

Ritchey and his corps of assistants. Few opticians would care to accept such a task, but no other man has had such experience in similar work as Professor Ritchey, and Mr. Hooker and Professor Hale have every confidence in his ability to complete the lens which is to unveil a universe, three hundred times vaster than that revealed by the most powerful modern refractors. New buildings and new apparatus must be designed and erected. It will require a year's time to construct the mounting of the telescope, which will be made at the Union Iron Works. Lack of space will prevent a detailed description of the various operations necessary to make the finished mirror. mirror. The problem is to produce a concave paraboloid surface, eight feet four inches in diameter, shelving to a depth of one inch at the centre. The innumerable operations necessary to produce this result may be roughly grouped under the following five divisions: rough grinding; fine grinding; polishing; figuring; silvering. Figuring the paraboloid surface is the most important and difficult of these operations, as will be realized when it is known that 8,000 square inches of surface must be covered and that when finished there will be no error of form on any part of it larger than twomillionths of an inch.

The experience of Professor Ritchey and his assistants, obtained in their work on the lens of the great sixty-inch reflector, which is to go into commission at the Solar Observatory at once, has resulted in the introduction of many improved methods and appliances, which will be of great advantage in this larger undertaking.

It is the judgment of trained astronomers that no greater opportunity has ever been presented in the entire history of astronomy than will be afforded by the construction of this twentieth century reflector.

Nature Fights the Railroads

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By W. G. Fitz-Gerald

EASELESS battle with Nature's forces, the world's railroad builders must fight-battles infinite in variety, and with opponents ranging from elephants in India to earthquakes in Japan; with drifting grass in the Argentine; snow in Canada; floods in Mexico; locusts in Brazil; sand in Australia; hostile savages in Uganda who convert the rails into spears, and the telegraph lines into money and ornaments for their

women.

Man-eating lions stopped all work at one time on the Uganda Railroad, the

latest link in the "Cape-to-Cairo" system. At Mile 133, Tsavo Station, a pair of lions appeared on the river banks and terrorized the coolie workers for two months. When twenty-nine of them had been eaten by these cunning and resolute brutes, the coolies struck. Both lions were old, stiff in the limbs, worn of tooth, unable to pursue the larger antelopes which are their real prey. They displayed almost human intelligence in waylaying men and picking them out from the tents. The climax was reached when the lioness trotted up one day and grabbed a man off an open car just as the train was slowing into Tsavo. After that, there was nothing for it but to

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A TREMENDOUS LANDSLIDE ON THE LEOPOLDINA RAILROAD OF BRAZIL.

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