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larger one. Many of these, perhaps most of them, are in all probability 
matter which left the sun when the rest did and which are now for the first 
time brought near enough the earth to be permanently annexed. In a 
region where no large masses existed, the matter would continue to revolve in 
a finely divided state, such as we actually find in the zone of the minor planets. 
This zone lies between the orbits of Mars and Jupiter. In it have been found 
some 800 planets large enough to make a record on a photographie plate, and 
there is little reason to doubt that the whole number is many times greater 
aid the size of most of them so small that we can never see them except as 
they collectively make a faint band of light across the sky. In this zone 
we find what we should expect with small sizes—that is, very elliptical orbits 
and very high inclinations. One of these planets has an orbit of such eccen- 
tricity that while its mean distance is considerably greater than that of Mars, 
yet in one point in its orbit it comes much closer to the earth than any body, 
except the moon, and _ two others have perihelion distances less than that of 
Mars. 
Thus it is entirely possible that our planetary system resulted from a 
spiral nebula, and it is entirely possible that spirals may result from close 
approaches of two stars and we iay even say that it is all probable, at least 
more probable than any other plan yet proposed. 
There are still some difficulties. We must say that if our system resulted 
from a spiral, this spiral was not at all on the scale observed among the 
spirals in the sky. Such a nebula, having a radius equal to that of Nep- 
tune’s orbit, were it no farther away than the nearest star, would be a very 
insignificant object and might fail of detection entirely. At the probable 
distance of most of these objects it would certainly be invisible. We can see 
how a star might be torn apart so as to scatter material over a space the size 
of Neptune’s orbit, but the case 1s different when we consider some of the 
large spirals in the sky. The largest is known as the Great Nebula of 
Andromeda. It covers an are of over a degree in the sky. Assuming a 
parallax of 0/’.1, which is probably larger than the real value, this nebula 
from end to end must extend over a space more than 1,800 times the size of 
Neptune’s orbit, or 54,000 times the size of the Earth’s orbit. 
We have never determined accurately the distance of a single nebula and 
so do not know the real size of any one of them, compared to the solar system, 
but there is no reason to suppose they are nearer than many of the faint stars. 
If this is true, their volumns are vast beyond comprehension and their density 
an inconceivably small fraction of the density of our best vacuum. It has 
