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which the revolutions of the components of the system must 
occupy. Madler had already assigned to the revolution of 
Alcor around Mizar (Zeta Ursse) a period of more than 7000 
years. But if these stars, which appear so close to the naked 
eye, have a period of such length, what must be the cyclic 
periods of stars which cover a range of several degrees upon the 
heavens.” (From Zeta to Beta is a distance on the heavens of 
about nineteen degrees.) “ The peculiarities of the apparent 
proper motions of the stars,” I added, lend a new interest to 
the researches which Dr. Huggins is preparing to make into the 
stellar proper motions of recess and approach.” * 
But a few months later, in a lecture delivered at the Royal 
Institution I pointed out more definitely what result I expected 
from Dr. Huggins’s researches. “ Before long,” I said, “ it is 
likely that the theory of star-drift will be subjected to a crucial 
test, since spectroscopic analysis affords the means of determining 
the stellar motions of recess and approach. The task is a very 
difficult one, but astronomers have full confidence that in the 
able hands of Dr. Huggins it will be successfully accomplished. 
I await the result with full confidence that it will confirm my 
views.” f 
It will be manifest that if the five large stars in Ursa are 
really travelling in the same direction, then, when Dr. Huggins 
applied the new method of research, he would find that, so far 
as motion in the line of sight was concerned, these stars were 
either all receding or all approaching at the same rate, or else 
that they were all alike in showing no signs of any motion, 
either of recess or approach. 
But in the meantime there was another kind of evidence 
which the spectroscope might give, and on which I formed some 
expectations. If these stars form a single system it seemed 
likely that they would all be found to be constituted alike — in 
other words, that their spectra would be similar. Not indeed 
that associated stars always display such similarity. Indeed 
the primary star of a binary system not unfrequently exhibits 
a spectrum unlike that of the small companion. But the five 
large stars in Ursa, being obviously primary members of the 
scheme they form, might be expected to resemble each other 
in general constitution. Moreover, since the stars not included 
in the set — viz., Alpha and Eta — might be regarded as probably 
very much nearer or very much farther away, it was to be ex- 
pected (though not so confidently) that these two stars would 
have spectra unlike the spectrum common (on the supposition) 
to the five stars. 
* “Proceedings of the Royal Society,” Jan. 20, 1870, pp. 170, 171. 
t “Report of the Royal Institution of Great Britain,” Weekly Evening 
Meeting, Friday, May 6, 1870, p. 7. 
