648 
POPULAE SCIENCE EEVIEW. 
which have been broken up by the attraction of the sun, and which have 
thus, in the course of years, become isolated members of the solar system. 
He is further confirmed in this view by the fact that the orbits of the above 
three comets intersect each other at the same points of the heavens, which 
should also take place, as, when the original mass first comes under the influence 
of the sun, it has a rectilinear motion directed to a certain point in the sky, 
and the plane of the new orbit must pass through this point. M. Hoek is 
of opiuion that those comets might formerly have revolved round some other 
star, and recommends careful spectrum analysis of all the comets which 
appear. 
Bearings of Transit Pivots. — Lieut.-Col. Strange recommends, instead of 
the ordinary Y bearings (as the form of a semicircular bearing is quite un- 
attainable) that of the segment of a circle, or a semicircular bearing in which 
a broad groove is cut away in the lowest part, so that the pivot rests upon two 
segments of a circle. This he considers far preferable to the ordinary angular 
bearings ; it has been adopted by Continental and British makers. Tlie 
latter is difi&cult to make properly, and cuts a groove in the pivots, which 
prevents the determination of the level error, whilst the wear and tear is ex- 
cessive, and alters their circular form. 
Eye-'j)iece for diminishing the Apertures of Telescopes. — Mr. Dawes de- 
scribes an instrument for this purpose, by which it can be done without dis- 
turbing the telescope, in order to apply a diaphragm of the required dimen- 
sions to the object-glass. It consists of an eye-piece, furnished vith a revolving 
diaphragm, containing the apertures of different sizes, each of which can be 
brought into the centre of the field of view. By sliding one of those aper- 
tures towards or from the object-glass, the cone of rays is respectively reduced 
or augmented, which has the same effect as diminishing or increasing the 
aperture of the object-glass. A scale affixedshows the amount of the diminu- 
tion of the aperture. Mr. Dawes has also applied sliding-wedges of neutral 
tint glass to his solar eye-piece ; and one of the great advantages of this 
method is, that in the case of double stars the larger may be excluded from 
the field of view (the small aperture of the field of the solar eye-piece 
enabling this to be done), when the brightness of the faint one may be deter- 
mined. Thus the companion of Alpha Lyrce, when seen beside the large 
star, is only of the 11*2 magnitude, but excluding it, rises to the 9 ‘7 magnitude. 
The comparative brightness of the different parts of the moon, and of the 
surface of the sun with our niummated atmosphere, may thus be determined. 
The values of the scales attached to those wedges have been accurately deter- 
miaed by means of stars whose magnitude had been previously found by 
means of the diminution of the aperture of the object-glass. 
Apjpearance of Mars. — During the last opposition of Mars, jVIr. Dawes 
perseveringly pursued his researches on the surface of that planet, directing 
his attention not only to the well-known spots but to those features which 
can only be suspected, or are less distinct. He used every precaution to 
keep the planet in view until those rare flashes of perfect vision occurred, 
and thus transferred them to the drawing ; finally comparing the latter with 
the original. Both the micrometer and eye-estimations were made use of, 
and the times noted. Some curious details were brought out, which he had 
not seen before. The most remarkable was a long narrow streak, running 
