1922.] The Ninth Indian Science Congress. L.8.C. 39 
gions. Extensive researches have been made at the Harvard 
Observatory for calibrating the scale of these magnitudes. The 
Harvard North Polar Sequence furnishes accurate determina- 
tions of the photographic magnitudes of stars round the North 
Pole, ranging from the brightest by continuous gradations 
down to the stars of the 21st mag. indicated by a light ratio of 
one to one hundred millionth. The sequence serves as an ex- 
cellent standard for comparing the magnitudes of stars in 
other regions. » Similar investigations have also been made ata 
number of other observatories notably at Géttingen, Mount 
Wilson and Yerkes for securing a uniform scale of photographie 
magnitudes. The difference between the photometric and the 
photographic magnitude is termed the colour Index; it is re- 
lated to the star’s spectral type and forms and excellent mea- 
sure of its colour. 
Counts of stars up to definite limits of magnitude give 
much information about the extent of the stellar system. Sys- 
tematic counts have been made in the vast amount of material 
Another important feature connected with the apparent 
distribution of stars is known as the Galactic concentration, 
i.e. the increase in star-density per unit area of the sky with 
