SECTIONAL TRANSACTIONS.— A. 383 



Friday, August 19. 



Presidential Address by Dr. C. G. Darwin, F.R.S., on Logic and 

 probability in physics fio.o). (See p. 21.) 



Prof. H. Shapley. — Metagalactic gradients and the expanding universe 

 hypothesis (11.20). 



The evidence for important metagalactic density gradients, which are of 

 sufficient magnitude to invalidate the assumption of uniformity throughout 

 the regions of space now attainable, is derived from surveys of the popula- 

 tion and distribution of galaxies within a radius of over one hundred million 

 light-years. The surveys cover extensive areas of the sky and display 

 large-scale structural features of the metagalaxy. 



The character of the inner metagalaxy is demonstrated by the distribution 

 of galaxies brighter than the thirteenth magnitude over the whole sky and 

 by preliminary results on the distribution down to the fifteenth magnitude 

 in the south galactic cap. The extension into southern declinations of the 

 great cluster of bright galaxies in Virgo adds information on large-scale 

 irregularity. Methods are shown of analysing distribution in both high 

 and low galactic latitudes. Regions near the Milky Way plane, in which 

 external galaxies are numerous, are studied for a determination of the extent 

 of absorption within our own galactic system. Surveys involving 200,000 

 galaxies in the equatorial and galactic polar caps are presented. 



Density differences between the north and the south sides of the Milky 

 Way are found to be considerable. The radial gradient discussed by 

 Hubble is compared with similar gradients across the sky, especially with 

 that which is found to extend over 125° across the south galactic cap. This 

 gradient presents such conspicuous density changes that it demonstrates the 

 impossibility of using the observed distribution of galaxies to derive a 

 coefficient of expansion of the universe. It is clear that such large-scale 

 irregularities are an important feature of the metagalaxy and must be 

 considered in cosmological theories. 



Prof. R. W. Wood. — Diffraction gratings for astrophysical purposes (12.0). 



Recent improvements in the technique of ruling gratings have made 

 possible the concentration of 85 per cent, of the incident light in the first 

 order spectrum, with a ruling of 15,000 lines to the inch. Two plane 

 gratings ruled on 8-inch aluminised pyrex discs, one concentrating in the 

 first, the other in the second order, are now in constant use in the spectro- 

 graph of the lOO-inch telescope at Mount Wilson, and have proved superior 

 to prisms, especially in the ultra-violet. 



Measurements have been made, with a photronic cell and monochromatic 

 light of various wave-lengths, showing the distribution of intensity for the 

 central image and various orders, at different angles of incidence. The 

 central image may contain as low as i per cent, of the total light. 



Large replicas have been made giving equally high concentration and 

 these are being used at the Harvard Observatory for the determination of 

 star colours, the grating, covered by a purple filter, being mounted a few 

 inches in front of the photographic plate. A new attachment to the dividing 

 engine makes possible the ruling of large concave gratings of very short 

 focus with a groove of constant shape over the entire area, thus abolishing 

 what I have called the ' target pattern ' (circular zones of low efficiency). 



