PROBLEMS OF MODERN SCIENCE 



with the accurate determination of the places of 

 the stars. The scale of the Franklin-Adams charts 

 is too small to lend itself to this purpose. On the 

 other hand, while the international plates do not 

 show stars fainter than the twelfth magnitude, the 

 Franklin-Adams limit is the seventeenth magnitude. 

 It may here be desirable to explain the precise 

 significance of the term ' magnitude ' as used in 

 Astronomy. From very ancient times the stars 

 have been arranged in order of brightness, the 

 brightest being said to be of the first magnitude, 

 while the faintest visible to the naked eye were of 

 the sixth. Stars of intermediate brightness were 

 assigned to one or other of the four remaining 

 magnitudes. The classification was a rough one, but 

 when the development of astronomical instruments 

 enabled differences in brightness to be observed 

 with a greater degree of precision, an exact defini- 

 tion of magnitude became necessary. It is there- 

 fore now universally agreed that if the amount of 

 light emitted by a star A is ioo times the amount 

 emitted by a star B, then the magnitude of B is 

 5 units greater than the magnitude of A. It is 

 to be noted that, contrary to the ordinary accepta- 

 tion of the term, increasing magnitude signifies, 

 not increasing brightness, but increasing dimness. 

 With regard to intermediate differences of magni- 

 tude, it may be remarked that one unit corre- 

 sponds to a light ratio of about 2^, so that, for 



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