300 
EASTMAN. 
Comets. 
The whole number of comets, real and suspected, from 
about 1770 B. C. to the end of 1889 A. D., the elements of 
whose orbits have not been computed, is 472. From 370 B. 
C. to the end of 1889 A. D. the number of comets the ele¬ 
ments of whose orbits have been computed is 309. Of these, 
18 are known to have elliptic orbits. In the case of 52, the 
computed elliptic orbits have not been verified by observa¬ 
tion. 
The computations show that 231 have parabolic orbits, and 
indicate that 7 have hyperbolic orbits. 
Thus it appears that not more than seven per cent, of the 
comets whose orbits have been discussed are known to have 
elliptic orbits, while it is almost certain that seventy-five 
per cent, have parabolic orbits. Of course, the periodic 
comets, whatever their origin, belong now to the solar system. 
As it is highly improbable that there are two or more 
kinds of comets of intrinsically diverse character and of 
different origin, it follows that all the comets had their 
genesis beyond the limits of the solar system, and that the 
few periodic comets are the exception to the general law, and 
at best are only adopted members of the solar family. 
There are only two sources of actual knowledge of the 
physical constitution of comets : 
One is from the use of the spectroscope; the other is the 
behavior of the light from a star when seen through various 
portions, but especially the nucleus of a comet. 
As is well known, observations with the spectroscope are 
not always easily interpreted, but in this case the difficulty 
is not so great as at first it seems to be. 
It is a general law that where there is a continuous spec¬ 
trum containing all the primary colors without gaps, the 
light is derived from an incandescent solid or liquid body. 
A discontinuous spectrum containing bands or bright lines 
indicates that the light comes from luminous gases or vapors. 
