152 
ASTRONOMY: SHAPLEY AND MA YBERRY Proc. N. A. S. 
the percentages of new Globes that have occurred from setting the various 
mutants and from crossing them reciprocally with normals. The percent- 
age of Globes appears to be somewhat higher than from the main line par- 
ents. It is of interest to note that when mutants are used as males in 
crosses with normals, the percentage of Globe mutations is reduced to about 
the average expected from the assemblage of main line parents. A com- 
parison of table 3 with table 1, however, will be instructive and lead us to 
the conclusion that the egg cell is not only more effective in transmitting 
the mutant character, once it has arisen, but is also more effective in 
originating these mutations than is the pollen grain. 
^lakeslee, A. F. and B. T. Avery, J. Heredity, 8, 1917 (125-131). 
2 Blakeslee, A. F. and B. T. Avery, Ibid., 10, 1919 (111-120). 
3 Blakeslee, A. F., John Belling and M. E. Farnham, Science, N. S., 52, 1920 (388-390). 
STUDIES OF MAGNITUDES IN STAR CLUSTERS 
XIII. VARIABLE STARS IN N. G. C. 7006 
By Harlow Shaplky and Beatrice W. Mayberry 
Mount Wilson Observatory, Carnegie Institution oe Washington 
Communicated by G. B. Hale, March 22, 1921 
The faintest and most distant variable stars on record have been found 
on photographs of the globular cluster N. G. C. 7006 made with the 60- 
inch and 100-inch reflectors at Mount Wilson. The new variables are 
of considerable value in measuring the size of the galactic system, because 
they are members of one of the remotest stellar groups now known and 
serve to determine its distance and dimensions. 
The position of the cluster for 1900.0 is: 
R.A. = 20 / *56 m .8, Decl. = +15° 48' 
Its galactic coordinates are: 
(3 = -20°, X = 32° 
and its apparent diameter on the Franklin-Adams charts is three-fourths 
of a minute of arc. 
By the usual photometric methods for determining the distances of 
globular clusters, the parallax of N. G. C. 7006 has been estimated to be: 
tt= 0". 000015, from the parallax-diameter correlation. 1 
7r= 0.000014, from photographic magnitude of brightest twenty -five stars. 2 
tt= 0.000016, from photovisual magnitude of brightest thirty-eight stars. 3 
7r= 0. 000016, from integrated visual magnitude. 4 
The foregoing accordant results make it fairly certain thatN. G. C. 7006 
is somewhat more distant than any other globular cluster in the present lists ; 
