46 



NATURE 



[September 9, 19 15 



Photometry and Colour. 

 For the more distant parts of the heavens proper 

 motions are an uncertain guide, and we must depend 

 on what can be learned from the light of the stars 

 by means of stellar photometry, determinations of 

 colour, and studies of stellar spectra. Speaking 

 generally, we attempt to discover from the nearer stars 

 sufficient about their intrinsic luminosities to enable 

 us to use the apparent magnitude as an index of the 

 distances of the stars which are further away. The 

 most striking example is found in Prof. Hertzsprung's 

 determination of the distance of the small Magellanic 

 cloud. From a knowledge of the characteristics of 

 the Cepheid variables found in this cloud by Miss 

 Leavitt, and their apparent magnitude, he deduced 

 the distance of the cloud as 10,000 parsecs. 



Much attention has been given of late years to 

 stellar photometry. In 1899 Prof- Pickering published 

 the Revised Harvard Photometry giving the mag- 

 nitudes of all stars brighter than 6-5m. In 1907 

 Messrs. Miiller and Kempf completed a determination 

 of 14,199 stars of the northern hemisphere brighter 

 than 7-5m. In 1908 a catalogue of 36,682 stars fainter 

 than 6-5m. was published at Harvard. These deter- 

 minations derive additional importance as they give 

 the means of standardising estimates of magnitude 

 made by eye, particularly the many thousands of the 

 Bonn Durchmusterung. 



By the labours of Prof. Pickering and his colleagues 

 at Harvard, Prof. Schwarzschild, Prof. Parkhurst at 

 Yerkes, Prof. Seares at Mount Wilson, and others, 

 the determinations of the magnitudes of stars by photo- 

 graphy has made rapid strides. As yet no complete 

 catalogues of photographic magnitude corresponding 

 to the Revised Harvard Photometry have been pub- 

 lished, though considerable parts of the sky and 

 special areas, such as the Pleiades, have been carefully 

 studied. The determination of the photographic mag- 

 nitudes of any stars which may be required is, how- 

 ever, a comparatively simple matter when the mag- 

 nitudes of sufificient standard stars have been found. 

 A trustworthy and uniform scale has been to a large 

 extent secured by the use of extra-focal images, grat- 

 ings, and screens in front of the object glass, and the 

 study of the effects of different apertures and different 

 times of exposure. 



At Harvard and Mount Wilson, standard mag- 

 nitudes of stars near the north pole have been pub- 

 lished extending to nearly the twentieth magnitude. 

 In the part of the range extending from lo-om. to 

 i6om. these agree very satisfactorily. There is, how- 

 ■ever, a difference of o-4m. in the scale between 6 om. 

 and loom, which needs to be cleared up. 



A uniform and accurate scale of magnitude is of 

 fundamental importance In counts of the numbers of 

 stars. Such counts aim at the determination of two 

 things : (i) how the numbers vary In different parts 

 of the sky, and (2) what is the ratio of the number of 

 stars of each magnitude to that of the preceding 

 magnitude in the same area of the sky. The counts 

 of stars from the gauges of Sir William and Sir John 

 Herschel, those of the stars contained In the Bonn 

 Durchmusterung, those made by Prof. Celoria, and 

 the recent counts of the Franklin-Adams plates bv Dr. 

 Chapman and Mr. Melotte, all agree in showing a 

 continuous increase of stars as, we proceed from the 

 pole of the galaxy to the galaxy itself. The Import- 

 ance of this fact Is that it shows a close connection 

 between the Milkv Way and the stars nearer to us. 

 The Milky Way is not a system of stars beyond the 

 others, but is the primary feature of our "Island 

 universe." 



Photometric observations have acquired additional 

 importance from the differences between photographic 



NO. 2393, VOL. 96] 



and visual magnitudes. The ordinary plate Is more 

 sensitive to blue light than the eye, and the difference 

 between the photographic and visual (or photo-visual) 

 magnitude of a star is an index of the colour. The 

 colour index Is found by observation to be related 

 very closely to the type of spectrum. Prof. Seares has 

 shown from the colour indices that as the stars become 

 fainter they become progressively redder. Prof. Hertz- 

 sprung has found the same thing by the use of a 

 grating in front of the object glass. Among stars of 

 170m. visual magnitude, Seares found none with a 

 colour index less than 07 ; this is approximately the 

 colour index of a star of solar type, i.e. near the 

 middle of the range from blue stars to red stars. 



There are three ways in which this may occur. The 

 stars may be bright but very distant red stars ; or they 

 may be faint red stars, like those In the Immediate 

 neighbourhood of the sun ; or there may have been an 

 absorption of blue light. It Is not possible to say in 

 what proportion these causes have contributed. The 

 red stars of 9-om. and loom. are nearly all very 

 luminous but distant bodies, but it seems likely that 

 stars of 170m. will contain a greater proportion of 

 stars of small luminosity. 



The absorption of light in space is very small, and 

 as yet imperfectly determined. Prof. Kapteyn and Mr. 

 Jones, by comparing the colour Indices of stars of large 

 and small proper motion, make the difference between 

 the absorption of photographic and visual light as 

 im. In 2000 parsecs. The question has been examined 

 directly by Prof. Adams, who has obtained spectra 

 of near and distant Stars which are identical as regards 

 their lines, and has examined the distribution of the 

 continuous light. This direct method of coitiparison 

 showed that the more distant star was always weaker 

 in violet light. But as both these investigations show 

 that very luminous stars are intrinsically somewhat 

 bluer than less luminous stars of the same spectral 

 type, the two causes require further research for their 

 disentanglement. The question is of Importance, as 

 it may serve in some cases to determine the distances 

 of very remote bodies the type of spectrum of which 

 is known. 



It must be admitted that we are as yet very ignorant 

 of the more distant parts of the "island universe." 

 For example, we can make little more than guesses 

 at the distance of the Milky Way, or say what part 

 is nearest to us, what are its movements, and so on. 

 But, nevertheless, the whole subject of the construc- 

 tion of the heavens has been opened up in a remarkable 

 manner In the last few years. The methods now em- 

 ployed seem competent to produce a tolerably good 

 model showing the co-ordinates and velocities of the 

 stars as well as their effective temperatures and the 

 amount of light they radiate. Industry in the collec- 

 tion of accurate data Is required, along with constant 

 attempts to Interpret them as they are collected. The 

 more accurate, and detailed our knowledge of the 

 stellar system as it is now, the better will be our 

 position for the dynamical and physical study of its 

 history and evolution. 



NOTES. 



The director of the Meteorological Office report.* 

 that information has been received from the Seismo- 

 logical Observatory at Eskdalemuir, Scotland, of the 

 record of a large earthquake which occurred at i a.m. 

 on Tuesday, September 7. The computed position 

 of the epicentre is lat. 9° N., long. 86° W., with a 

 possible error of 10°. The position mentioned is in 

 the Pacific Ocean, about 70 miles from Cape Blanco 



