PRESIDENTIAL ADDRESS. 365 



it quite clear what this scale means, that for every increase of 5'0 m. there 

 is a diminution of light in the proportion of 100 :1. Thus the total range in 

 going from the brightest stars to those of 20-0 m. is more than 10" to 1. 



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 : (1) 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 con- 

 tained in the Bonn Durchmusterung, those made by Prof. Celoria, and the 

 recent counts of the Franklin-Adams plates, all agree in showing a continuous 

 increase of stars as we proceed from the pole of the Galaxy to the Galaxy 

 itself. The importance of this fact is that it shows a close connection between 

 the Milky Way and the stars nearer to us. The Milky Way is not a sy.stem 

 of stars beyond the others, but is the primary feature of our 'island universe." 



So far there is general agreement. Depending mainly on the counts of Sir 

 John Herschel made at the Cape, and the Cape Photographic Durchmusterung, 

 Prof. Kapteyn finds a very great concentration of faint stars towards the 

 Milky Way. On the other hand, the Bonn Durchmusterung, the counts of 

 Prof. Celoria, and the recent counts of the Franklin-Adams plates by Mr. 

 Chapman and Mr. Melotte give nearly the same concentration, e.g., the pro- 

 portion of 16"0 m. to 90 m. stars does not vary much at different distances from 

 the Galaxy. According to these counts, the total number of stars brighter 

 than 6-0 m. is approximately four times the number brighter than 50m. in all 

 parts of the sky ; the number brighter than 90 in. is three times the number 

 brighter than 8-Om. ; the number greater than 15-0 m. is double that greater 

 than 14-0 m. From the gradual diminution of the ratio for successive magni- 

 tudes, the total niunber for the whole sky is inferred to be between 1,000 

 and 2,000 millions, the median coming about the magnitude 23-0 m. The total 

 amount of light received from all the stars is equivalent of 700 or 800 stars of 

 the first magnitude, of which half comes from the stars brighter than 100m. 

 These counts do not by themselves make it possible to determine how the stars 

 fall off in density. But as Prof. Eddington has pointed out, they give a 

 measure of the flattening of the stellar system in the ratio of 3^ to 1. If 

 there is a concentration of faint stars in "the Milky Way, as maintained by 

 Prof. Kapteyn, this ratio will be increased. 



Photometric observations have acquired additional importance from the 

 differences between photographic 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. Hertzsprun"- 

 has found the same thing by the use of a grating in front of the object glass" 

 Among stars of 170 m. visual magnitude, Seares found none with a colour index 

 less than •7; this is approximately the colour index of a star of Solar type 

 t.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 

 imniediate neighbourhood of the Sun ; or there may have been an absorption 



plue light. It IS not possible to say in what proportion these causes have 

 contributed. The red stars of 9-0 m. and 100m. 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 vet imperfectly deter- 

 mined. Prof. Kapteyn and Mr. Jones, by comparing the colour indices of 

 stars of large and small proper motion, make the difference between the absorp- 

 tion of photographic and visual light as 1 m. in 2,000 parsecs. Tlie question has 

 been examined directly by Prof. Adams, who has obtained spectra of near 

 and distant stars whicli are identical as regards their lines, and has examined 

 showeil f Iv.f nli ^'^^,.7"t'nuous light This direct method of comparison 

 showe<I that the more distant star was always weaker in violet li-Iit But -is 



