SCIENTIFIC SUMMARY. 
189 
the compound prism. On turning the micrometer screw, the slide which 
holds the glass plate travels in grooves, and the illuminated cross is thus 
made to traverse the whole length of the spectrum. It may be suggested, 
that perhaps a single short line, at right angles to the direction of the 
spectral lines, might serve as well as the cross-lines, if only its centre were 
marked with a dark point. By such an arrangement the dark line to be 
measured would not, however faint, be obliterated near its centre, as when 
the angle of the cross-lines falls there. 
Application of Photography to the coming Transits of Venus. — Mr. 
Proctor’s paper on this subject, in the Monthly Notices for January, 
points out the advisability of combining certain pairs of stations, so chosen 
that the parallactic displacement of Venus as seen from those stations may 
be on a radial line of the sun’s disc. In all other cases, he remarks, an error of 
importance may be expected to accrue from errors in superposing the fidu- 
cial lines of two photographs which are to be compared. Where the dis- 
placement is on a radial line there is no necessity for superposition, since the 
mere measurement of the planet’s distance from the centre of the sun’s disc 
will afford the necessary information. The large plate which illustrates 
Mr. Proctor’s paper has been very skilfully engraved by Messrs. Malby and 
Sons. 
The New Eye-piece for correcting Atmospheric Dispersion. — A very simple 
arrangement for getting rid of the flint prisms proposed by Mr. Airy, with- 
out introducing optical defects, occurred independently to the Astronomer 
Royal and to Mr. Simms. It consists merely in making the eye-glass — 
plano-convex — broader than is strictly necessary for telescopic vision, caus- 
ing it to press by its convex surface into a concave cup at the eye end of 
the eye-piece, and allowing it to roll in that concavity j thus presenting 
different parts of its convex surface, though always in the same form and 
position, to the rays of light which come from the field-glass, but presenting 
to the eye a plane surface, which, in one position of the lens, is normal to 
the telescopic axis, and in other positions is inclined to it at different 
angles. 
New Theory of the Milky Way. — Mr. Proctor has been led, by a careful 
examination of the structure of various parts of the Milky Way, to the con- 
clusion that the true figure of the system of stars constituting this zone can 
neither be that of a cloven disc, as supposed by Sir W. Herschel, nor that 
of a broad flat and in part cloven ring, as suggested by Sir John Herschel. 
He points out that the existence of round coal-sacks in the Milky W ay is 
as conclusive as to its figure, at least in those parts, as the round figure of 
the Magellanic Clouds is as to the general figure of those strange clusters. 
We cannot suppose the coal-sacks to be tunnel-shaped openings extending 
through the whole breadth of a wide flat ring, without the ({ obvious im- 
probability” spoken of by Sir John Herschel when dealing with the suppo- 
sition that the nubeculae may be cylindrical in figure. This being so, it 
follows that if the coal- sacks are really openings through a star-zone, that 
zone cannot, in all probability, have a much greater extension in the direc- 
tion of the line of sight than at right angle's to that line. According to this 
view the section of the Milk'y Way near the coal-sack in Crux (and pre- 
sumably elsewhere) would be, roughly, circular. And viewing the Milky 
