242 CARNEGIE INSTITUTION OF WASHINGTON. 



feet deep in the center, tapering towards the ends. Its cross-section at the 

 center is 4| feet, tapering to a width of 2| feet at a point 18 feet from the center, 

 and maintaining this width to the extremity of the girder. On the upper 

 surface of the girder, which is approximately straight, the rails which carry the 

 sliding mirrors, carefully planed in sections 12 feet in length (the limit of our 

 planer-bed), are supported by leveling screws, which permit them to be accu- 

 rately alined. The outer 45° plane mirrors, 15 inches in diameter, are mounted 

 on carriages moved simultaneously toward or away from the center of the 

 girder by long screws driven from a single motor. Their separation may 

 thus range from 7 to 50 feet. To permit stars of any declination to be reached, 

 the 45° mirrors can be rotated simultaneously, by synchronous motors, about 

 the axis joining their centers. The inner 45° plane mirrors, also 15 inches 

 in diameter, fixed in position 26 inches apart on opposite sides of the center 

 of the girder, are provided with fine adjustments, and one of them can be 

 moved slightly for compensation of path. The light received from the outer 

 mirrors is reflected to a paraboloidal mirror of 36 inches aperture and about 

 15 feet focal length, mounted within the girder, at its base. Since the axis 

 of the mirror is normal to that of the girder, the rays are reflected back between 

 the fixed 45° mirrors to a diagonal plane mirror, which sends them to an eye- 

 piece (directly toward the pole) conveniently placed for the observer, who 

 sits on a platform attached to the girder on the north. 



The girder is carried by a strong polar axis, consisting of a short steel 

 forging mounted on standard roller bearings, supported on a massive pier of 

 concrete. The center of the axis passes through the center of gravity of the 

 girder, which thus remains in balance in all positions. The end-thrust is 

 carried partly by the bearings of the polar axis and partly by two auxiliary 

 rollers moving on curved rails on the north face of the pier, opposite the 36- 

 inch mirror. A worm-gear sector of 10 feet radius, bolted to the girder, is 

 driven by a worm connected with a driving-clock at the base of the pier on 

 the north. The range of motion is 1^ hours on each side of the meridian. 



Optically the instrument is similar to the 20-foot interferometer, except 

 that a 36-inch mirror replaces the central zone of the 100-inch, while provision 

 is made for motion in declination by the simple expedient of rotating the outer 

 45° mirrors. Comparison fringes will also be arranged for, and the wedge 

 for compensating differences in path will be like that of the 20-foot interfer- 

 ometer. The new instrument will be covered by a house with double walls 

 of steel, about 60 feet long. 18 feet wide, and 20 feet high. The lower walls, 

 to a height of 8 feet, will be fixed in position, but the upper section of the 

 house may be rolled back, leaving the interferometer fully exposed for obser- 

 vations from declination —30° to the pole. 



With this instrument, which is now under construction, it should be possible 

 to check the results obtained with the 20-foot interferometer by simultaneous 

 observations, and to measure the diameters of more than 30 stars brighter 

 than the fourth magnitude. 



THE VELOCITY OF LIGHT. 



The stations used by Professor Michelson during the past summer for his 



investigations of the velocity of light were situated on Mount Wilson and on 



the ridge of Mount San Antonio, at a distance of about 23 miles. The Mount 



Wilson station was moved to a point just south of the power-house, where 



