technique. Nevertheless many physicists might have hesitated, despite this simplification, at the idea of causing two bundles of rays to interfere which at their start were situated 5 or 10 meters apart. For approaching this startling feat, the audacity of the American physicist, his profound knowledge of optics and long experience with the most delicate apparatus was necessary.

The first interferometer, constructed, in 1920, had the two receiving mirrors 6 meters apart. They were mounted upon the Hooker telescope (fig. 4), not because of its optical power, which was not

FIG. 4.-The 2.50 meter telescope provided with the
interferometer.

The light, reflected by the mirrors, M1, M2, M3, and
M. of the interferometer upon the great parabolic
mirror a, is then reflected to the hyperbolic mirror b
(the telescope is mounted as a Cassegranian). A plane
mirror c, inclined at 45°, then sends the light to the
eye-piece d situated at the side of the tube. It is close
to this ocular that the compensating glasses are placed.

necessary, as we have just seen, but because of the advantages which accrued from its massive and rigid equatorial mounting, which admitted the extra load without accompanying trouble. Another interferometer of 16 meters, now actually under construction, will, on the contrary, be an independently mounted instrument.

We will give some details of the 6-meter instrument. The mirrors, 15 centimeters in diameter, are carried by a trussed girder. The two central mirrors are mounted 1.14 meters from each other. The other two can be reciprocally displaced. In order to accomplish this their supports are mounted on slide rests moved by two screws.

The simultaneous motion of the screws is accomplished by a small electric motor worked from a distance by the observer (fig. 5).

The method of observing consists in separating and approaching the first two mirrors and observing the sharpness of the fringes. When they become invisible the distance D of the mirrors M,M ̧ is recorded. The theory is not wholly identical with that of the experi

M

Мг

M3

M

901000000010901008

FIG. 5.-Detailed plan of the interferometer. M1, M2, M3, M1, plane
mirrors, mounted on moving carriages.

ment of Stéphan, so that his formula is not applicable. However, the formula for the angular diameter is still

w=1.22 X/D.

A readjustment is necessary after each displacement of the mirrors. For easing the labor of this, Michelson has devised various cunning contrivances without which the observations would have been excessively laborious if not impossible.

THE DIAMETER OF BETELGEUSE.

The decisive observation was made by Pease and Anderson on the 15th of December, 1920. The definition was fine and the images were observed without difficuty. The stars ẞ Persei, y Orionis, and a Canis Majoris gave fringes with D equalling 306 centimeters. But when the apparatus was trained upon Betelgeuse without changing the distance of the mirrors the fringes could not be seen. At a distance of 250 centimeters they appeared, although hazy. It was allowable then to call the value of D equal to 3 meters with an accuracy of about 10 per cent. We thence have

w=0'.047.

A restriction is, however, necessary. The theory postulates that the star disk is uniformly bright. It is more probably progressively

darker from the center outwards just as is the solar disk. Admitting that Betelgeuse has the same distribution of brightness as the sun, the value obtained from the formula must be increased by an amount which can not be exactly computed but which is about 15 per cent.

Michelson even proposes to determine the law of distribution of brightness upon the disk of Betelgeuse by a thorough study of the change in the sharpness of the fringes in approaching the minimum. Admitting for the present the value 0.047, let us get the linear diameter of the star. The best parallax from measures by several observers is 0.017. The diameter of Betelgeuse is therefore

0.047/0.017 2.7 astronomical units

300 solar diameters.

The equator of this giant star would thus contain the earth's orbit (two astronomical units in diameter) and nearly that of Mars (3.4 astronomical units.) That is in full accord with the earlier estimates.

The measurement of Arcturus made by Pease February 12, 1921, has also checked with the theoretical value. For this star it was necessary to use the total available length of the interferometer, 6 meters. This gives for Arcturus a diameter of 0'.024. The diameter predicted by Russell was 0.019. The parallax of the star is 0.095, according to van Maanen and Russell. Its diameter in linear measure is, therefore, 28 times that of the sun.

Finally, more recent observations have led to the value 0".040 for the angular diameter of Antares. The most probable parallax is 0.023. Whence we deduce a diameter for Antares 200 times larger than the sun's. Again the order of magnitude predicted is confirmed.

ATOMIC WEIGHTS AND ISOTOPES.1

By F. W. ASTON, M. A., D. Sc., F. R. S.,
Fellow of Trinity College, Cambridge.

That matter is discontinuous and consists of discrete particles is now an accepted fact, but it is by no means obvious to the senses.

FIG. 1.-Cubes 11 to 15 compared with familiar objects to scale.

The surfaces of clean liquids, even under the most powerful microscope, appear perfectly smooth, coherent, and continuous. The

Abstract of a summary of a series of lectures given before The Franklin Institute, March 6-10, 1922. Reprinted by permission from the Journal of the Franklin Institute, May, 1922.

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