408 New Goniometer Hye-piece for the Microscope. 
the positions of 1819, 1858 and 1869, Prof. Oppolzer finds for 
the first interval the value of the mean motion 638” 6312, and 
for the second interval 638’:7007. ‘This difference is so small 
that we may safely conclude that the comet’s motion is strictly 
in obedience to the law of gravitation. 
ence, so far as the motions of comets have been determined, 
the evidence is against the theory of a resisting medium in space. 
Thus far, the observations of the planets lead to the conclusion, 
that their motions are in strict accord with the law of gravita- 
tion; and in the disputes about the acceleration of the mean 
motion of the moon, no one has thought to seek its cause in a 
resisting medium, but much more probable causes are at hand. 
Encke’s comet, therefore, stands alone in the strange anomaly in 
its motion which the calculations have shown. The first thing 
to be done would be to test the correctness of these calculations; 
and for this purpose it seems to me that the method of speci 
“polseeagiras is better than the expansion of the general pertur- 
ations, since by the first method all powers of the disturbing 
all ot . 
pointed out, by Olbers I think, that this comet moves throu 
those regions where the zodiacal light is seen. Possibly also the 
numerous meteoric streams which are moving around the sun, 
and which are closely connected with the orbits of some of the 
comets, may exert an influence on their motions. 
Sept. 25, 1871. 
Art. LL—On a new Micrometric Goniometer eye-piece for 
the Microscope ; by J. P. SourHwortTH. 
ieeels Meee 
Saar SS ee retina ear 
AFTER a few experiments by Dr. H. T. Porter and myself, 
! ucceeded in i i 
we have su 
making an eye-piece micrometer and 
process are 
