II. Attention is drawn to the following recent papers bearing on "Photo-Electricity."

Foote and Mohler, P. R., 17, 394(1921), find the following ionizing and radiating potentials.

Joly, P. M., 41, 289 (1921) and Poole, P. M., 41, 347 (1921), discuss the application of photo-electricity and the quantum theory to vision. Experiments on the photo-electric effect of the active materials of the retina are described.

Stebbins, A. P. J., 53, 105 (1921), employed a photo-electric cell to study the fluctuations of the star Algol.

Angerer, P. Z., 22, 98 (1921), made use of a photo-electric cell to study the after glow in active nitrogen. In connection with the point raised by Elster and Geitel (p. 107), Angerer was able to study rather rapid changes of light intensity satisfactorily.

Sir J. J. Thomson, P. M., 41, 526 (1921), calculated the ionizing potentials for a number of elements on the basis of his theory of the atom. The ratio of the ionizing potential for Li to that for H is of the right order; the ionizing potential for Na appears to be considerably smaller than the experimental value. The theoretical

values for O and N cannot be compared directly with the experimental values which relate to O2 and N2.

A good discussion of critical potentials with reference to the Bohr type of atom will be found in Sommerfeld's "Atombau und Spektrallinien" (2nd edition). (The writer was unable to get a copy in time to make use of it in drawing up the report.)

A full account of the work on hydrogen referred to on p. 139 under the names of Franck, Knipping and Kruger will be found in an article by Kruger in A. d. P., 64, 288 (1921).

Hodgman, P. R., 17, 246 (1921), gives a list of color filters (dyes in gelatine) with transmission ranges (mainly in the visible), which may prove useful in photo-electric experiments. Data regarding the transmission of various colored glasses for use in securing monochromatic light when used in conjunction with the mercury arc, or the discharge in hydrogen or helium will be found in Technologic Paper No. 148, 1920 of the Bureau of Standards.

EVOLUTION OF THE IDEA OF GALACTIC SIZE

The physical universe' was anthropocentric to primitive man. At a subsequent stage of intellectual progress it was centered in a restricted area on the surface of the earth. Still later, to Ptolemy and his school, the universe was geocentric; but since the

*This address and the following one by Dr. Heber D. Curtis are adapted from illustrated lectures given on the William Ellery Hale Foundation before the National Academy of Sciences, April 26, 1920. The authors have exchanged papers in preparing them for publication in order that each might have the opportunity of considering the point of view of the other.

'The word "universe" is used in this paper in the restricted sense, as applying to the total of sidereal systems now known to exist.

time of Copernicus the sun, as the dominating body of the solar system, has been considered to be at or near the center of the stellar realm. With the origin of each of these successive conceptions, the system of stars has ever appeared larger than was thought before. Thus the significance of man and the earth in the sidereal scheme has dwindled with advancing knowledge of the physical world, and our conception of the dimensions of the discernible stellar universe has progressively changed. Is not further evolution of our ideas probable? In the face of great accumulations of new and relevant information can we firmly maintain our old cosmic conceptions?

As a consequence of the exceptional growth and activity of the great observatories, with their powerful methods of analyzing stars and of sounding space, we have reached an epoch, I believe, when another advance is necessary; our conception of the galactic system must be enlarged to keep in proper relationship the objects our telescopes are finding; the solar system can no longer maintain a central position. Recent studies of clusters and related subjects seem to me to leave no alternative to the belief that the galactic system is at least ten times greater in diameter-at least a thousand times greater in volume-than recently supposed.

Dr. Curtis, on the other hand, maintains that the galactic system has the dimensions and arrangement formerly assigned it by students of sidereal structure-he supports the views held a decade or so ago by Newcomb, Charlier, Eddington, Hertzsprung, and other leaders in stellar astronomy. In contrast to my present estimate of a diameter of at least three hundred thousand lightyears Curtis outlines his position as follows:2

As to the dimensions of the galaxy indicated by our Milky Way,till recently there has been a fair degree of uniformity in the estimates of those who have investigated the subject. Practically all have deduced diameters of from 7,000 to 30,000 light-years. I shall assume a maximum galactic diameter of 30,000 light-years as representing sufficiently well this older view to which I subscribe though this is pretty certainly too large.

I think it should be pointed out that when Newcomb was writing on the subject some twenty years ago, knowledge of those special factors that bear directly on the size of the universe was extremely fragmentary compared with our information of to-day. 1See Part II of this article, by Heber D. Curtis.

2Quoted from a manuscript copy of his Washington address.