October y, 1922] 



NA TURE 



48; 



flector. Immediately after the first successful measures 

 by Mr. Pease, both he and I made several designs 

 of large interferometers with independent equatorial 

 mountings, but their cost would have been too great to 

 warrant their construction. It was also thought ad- 

 visable to postpone further instrumental developments 

 until thev could be undertaken in the light of prolonged 

 experience with the 20-foot interferometer. 



The method has since proved so successful, and its 

 wider application so desirable, that the mechanical 

 problem has recently been taken up anew. Optically 

 the 20-foot instrument leaves nothing to be desired. 

 The new instrument is therefore simply a larger Michel- 

 son stellar interferometer adapted for the observation 

 of fainter and smaller stars, embodying no new optical 

 features, but carried by a mounting so simplified in 

 design as to reduce the cost of construction to a mini- 

 mum. My specifications for the mounting, which have 

 been improved in certain respects and developed into 

 working drawings by Mr. Pease and his associates in the 

 Division of Instrument Design of the Mount Wilson 

 Observatory, call for a light but very rigid skeleton girder 

 about 54 feet long and 10 feet deep at its centre, where its 

 cross-section is about 4^ feet (Figs. 1 and 2). This is to 

 be built of standard steel shapes, cut to length at the 

 mill and riveted together on Mount Wilson. The girder 

 will be bolted to a heavy plate carried by the upper ex- 

 tremity of the polar axis, which is a short steel forging 

 turning in standard roller bearings, mounted on the 

 upper face of a massive concrete pier. The polar axis 

 passes through the centre of gravity of the girder, thus 

 assuring its balance in all positions. A worm-gear 

 sector of long radius, bolted to the girder, is driven 

 by a worm connected with a driving-clock fixed near 

 the north face of the pier. The range of motion in 

 right ascension is ij hours east and west, thus allowing 

 ample time for the observation of a star when near its 

 meridian passage. 



The optical parts comprise a paraboloidal mirror of 

 36 inches aperture and about 15 feet focal length, 

 mounted within the girder, as shown in the illustrations. 

 The two outer plane mirrors, each 15 inches in diameter, 

 mounted at 45 on carriages which slide along rails 



bolted to the upper face of the girder, receive light from 

 the star and reflect it to two similar 45° plane mirrors, 

 fixed in position above the 36-inch mirror, to which 

 they send the two parallel beams. These are returned 

 as converging beams toward the focus, but are inter- 

 cepted by a (Newtonian) 45 plane mirror above the 

 centre of the girder, which sends the light to the focal 

 plane, in the direction of the north pole. The observer, 

 seated on a platform carried by the girder, makes the 

 necessary adjustments and determines the visibility 

 of the interference fringes corresponding to various 

 settings of the outer 45° mirrors, which are periodically 

 moved apart by a single long screw driven by an electric 

 motor. The distance between these mirrors, when the 

 fringes disappear completely, gives the angular diameter 

 of the star if the mean wave-length of its light is known. 



To reach stars north or south of the equator, the 

 two outer 45 mirrors are rotated simultaneously by 

 synchronous motors about the axis joining their centres. 

 In this way any star from the pole to 30 south declina- 

 tion can be observed when near the meridian. 



Throughout the design precautions have been taken 

 to reduce the amount of large and expensive machine 

 work to a minimum. The girder need be only approxi- 

 mately straight, as the rails, carefully planed in 12-foot 

 lengths (the limit of our planer bed), will be optically 

 lined up by adjusting screws. The final compensation 

 for length of path will be effected by a sliding wedge, of 

 the type designed by Prof. Michelson for the 20-foot 

 interferometer. Comparison fringes, adjustable for 

 visibility, will be provided as an aid to the observer. 

 The instrument will be covered when not in use by a 

 sheet steel house with double walls, the upper part of 

 which can be rolled away longitudinally by an electric 

 motor. 



This interferometer should permit the measurement 

 of more than thirty stars brighter than the fourth 

 magnitude, representing a wide range of spectral types. 

 It is now under construction in the instrument and 

 optical shops of the Mount Wilson Observatory. 2 



2 For a brief account of the 20-foot interferometer and its method of opera- 

 tion, see the chapter on " Giant Stars " in the writer's recent book " The 

 Xt'\v Heavens." reviewed in Nature of July 1 1, p. 2. Full details are given 

 by Messrs. Michelson, Pease, and Anderson in the Astrophysical Journal. 



Motorless or Wind Flight. 



By Dr. S. Brodetsky. 



RECENT achievements in motorless flight, variously 

 designated as gliding, soaring, and sailing, have 

 attracted considerable attention, and much discussion 

 has arisen as to the practical and military value of 

 this new development, as well as to its scientific 

 significance. While many authorities anticipate 

 nothing more than the emergence of a new " sport," 

 and ascribe little importance to motorless flights, 

 others of a more imaginative turn of mind foresee 

 great possibilities in this type of aerial navigation. 

 The motorless flying machine has even been pro- 

 claimed as heralding the doom of the engine-driven 

 aeroplane ! 



It is certainly premature to attempt a forecast of 

 the future of flight in a glider. The art of gliding is. 

 of course, older than that of flight in an engine-driven 



NO. 2762, VOL. I io] 



machine : Lilienthal's experiments with gliders were 

 made more than a generation ago, long before any 

 aeroplane containing a motor rose into the air and 

 executed a real "flight. But Lilienthal, Pilcher. 

 Chanute, Orville Wright, and others were not able 

 to stay aloft in a glider more than a few minutes ; 

 whereas during the recent competitions in German}-, 

 Martens remained in the air nearly three-quarters of 

 an hour, and Hentzen stayed in the air two hours, 

 and later three hours, performing evolutions of 

 an intricate character. It is therefore clear that 

 the art of gliding has entered upon a new phase, 

 and the scientific" problems involved merit careful 

 discussion. 



As already indicated, there is considerable diversity 

 in the names sriven to the flights thus carried out 



