OCTOBEB 21, 1921] 



SCIENCE 



389 



THE AMERICAN PHILOSOPHICAL 

 SOCIETY 



(Concluded) 



Measurement of star diameters hy the interfer- 

 ometer method: F. G. Pease. Stephan, in 1874, fol- 

 lowing a suggestion of Fizeau's, made an attempt 

 at measuring the angular diameter of the brighter 

 stars by the interferometer method and rightly 

 found that his telescope was too small. Miehelson 

 in 1890 devised a periseopie attachment, which, 

 when placed upon the end of the telescope greatly 

 increased its equivalent aperture; with it he meas- 

 ured the diameter of the satellites of Jupiter. A 

 similar instrument upon the great 100-inch Hooker 

 telescope on Mount Wilson enabled Miehelson and 

 Pease in 1920 to obtain a successful measure of the 

 angular diameter of a Orionis. 



A brief outline is given of the principles under- 

 lying the method of interferometer measurement, 

 and of its application to use with the telescope. 

 Pencils of Kght from the star passing through two 

 small apertures in a screen covering the telescope, 

 produce ' ' interference fringes ' ' which appear 

 superposed on the regular diffraction image of the 

 star. When the apertures are close together the 

 " visibility " of the fringes is said to be 100 per 

 cent. As the apertures are separated the visibility 

 is reduced, the fringes become weaker and at a 

 still further separation they vanish. The relation, 

 a =: 1.22 \/6 existing between the angular diameter 

 of a star, the effective wave length of its light and 

 the distance apart of the two outer mirrors when the 

 fringes vanish enables one to determine the star's 

 angular diameter. The linear diameter of the star 

 can then be calculated if its parallax is known. 

 The interferometer attachment is described to- 

 gether with the method of operating it. It consists 

 of a fabricated steel beam 21 feet long, carrying 

 four 6-inch mirrors, inclined at 45°, the two inner 

 ones being fixed and faced downwards, the two 

 outer adjustable and facing upwards. It is placed 

 on the end of the telescope and observations made 

 at the Cassegrain focus, which has an equivalent 

 focal length of 134 feet. An auxiliary optical de- 

 vice, consisting of a movable wedge of glass and a 

 plane parallel compensator enables the observer to 

 equalize the two pencils of light and obtain the de- 

 sired fringes. Two additional pencils passing over 

 the ordinary path in the telescope, form a compari- 

 son image with " zero " fringes superposed; both 

 interferometer and comparison images are viewed 

 simultaneously with an eyepiece. When the seeing 



is poor it is difficult to be certain that the fringes 

 have actually vanished; a weakening of the zero 

 fringes, however, at the same time furnishes the 

 ■observer with a cheek in the matter. 



On December 13, 1920, the interferometer fringes 

 vanished for a Orionis when the distance between 

 the mirrors was about ten feet. The seeing was 

 good and the instrument adjustments were verified 

 on check stars both before and after the observa- 

 tion. Assuming a wave-length of 5.75 x lO-s cm. the 

 approximate angular diameter is 0".047. Using a 

 value of 0".020 for the parallax, the linear diam- 

 eter is roughly 2,18,000,000 miles. 



Definite decrease in visibility of the fringes has 

 been observed by the writer with the 20 foot inter- 

 ferometer, for a Tauri, o Bootis, a Scorpii and (3 

 Geminorum. The diameter of /3 Geminorum is 

 smaller than can be measured with this interfer- 

 ometer. Additional observations will be necessary to 

 definitely determine the diameter of the others. The 

 work will be continued until all the brighter stars 

 have been examined. 



Atomic theory and ideal numbers: Leonard 

 Eugene Dickson. On the basis of close analogies 

 with the molecular and atomic theories, it is pos- 

 sible to give a clear insight into the nature of ideal 

 numbers, which play such an important role in the 

 mathematical world to-day. This special importance 

 is due to the fact that only after the introduction 

 of ideal numbers do the laws of divisibility, valid 

 in arithmetic, hold also for algebraic numbers. 

 Without ideal numbers the situation in regard to 

 algebraic numbers is most chaotic. The restoration 

 of order out of chaos by the invention of ideal num- 

 bers is one of the chief mathematical triumphs of 

 our century. 



A general catalog of stellar distances: Frank 

 Schlesingek. This paper deals with a review of the 

 various methods for determining stellar distances 

 and describes the methods that have been employed 

 to mold the observations into a homogeneous whole. 



Intermittent vision at low intensities: Herbert 

 E. Ives. An experimental study of the phenomena 

 of flicker at low intensities where twilight or rod 

 vision prevails. Blue light was used, reduced in 

 intensity until all sensation of color disappeared. 

 Under these conditions the speed of alternation of 

 light and dark at which flicker disappears, becomes 

 independent of changes of intensity, unlike its be- 

 havior at 'high intensities where it increases or de- 

 creases as the intensity is raised or lowered. The 



