toniadi and others, found on photographs of the star a halo, which did cot appear about other stars on the same plate, and which was thought to be nebulous. Later it was shown by Professor Max Wolf that this aureole was instrumental, and due to the fact that the Nova was rich in rays for which the lenses were uncorrected. At the same time Wolf found that the Nova was surrounded by a faint nebulosity. Long exposures with the powerful reflecting telescopes of the Yerkes and Lick Observatories showed well this nebulosity, and, especially, nebulous patches at considerable distances from the Nova. From later photographs it was announced from the Lick Observatory and confirmed at the Yerkes Observatory, that these nebulous masses are moving away from the Nova. This is a discovery of the highest importance, having a direct relation to the theory of new stars. The motion of the nebula is very great, amounting to about 1′ of arc in six weeks. Carried backward this motion would bring the nebulous masses at the Nova, when the outburst occurred, a fact of much interest. What this rate of movement represents in miles per second cannot be assumed safely until the star's parallax is known. This has not yet been determined. Our nearest neighbor among the stars, so far as known, has a parallax of less than 1″. A parallax as great as 1″ would indicate a velocity of something like 1,500 miles per second. Either the Nova is very near us, or else the velocity of the Nebula is almost inconceivably great. Indeed, if the parallax should prove to be too small for measurement, the fact would imply a velocity so great that it might be better explained as a motion of light, rather than of matter. The motion, moreover, appears not to be radial, but spiral. The broadening of the lines of the spectrum of the Nova furnishes a clue to the rapidity of motion, the value of which is, however, very doubtful. No definite conclusions can be safely drawn until more data arc obtained, and a satisfactory determination of the parallax is given. Meanwhile the astronomical world is watching the developments mth the keenest interest. Incidentally the investigation is furnishing a powerful argument for a more extended use of large reflecting telescopes. It may be, that the Golden Age of the refracting telescope has passed!
THE MAGNITUDE AND THE MASS OF THE VISIBLE UNIVERSE.
An interesting question which often occurs to the astronomer and the physicist is that of the magnitude and the material contents of the visible universe. While science is unable at present to give a decisive answer to this question it is nevertheless competent to correlate the observed facts to such an extent that a possible, if not a probable, answer is already attainable. The latest contribution to this subject is due to the indefatigable labors of Lord Kelvin. In the 'Philosophical Magazine' for August, 1901, he attacks the question from the dynamical side in an article 'On ether and gravitational matter through infinite space'; and at the September meeting of the British Association for the Advancement of Science he amplified his investigation in a paper on 'The absolute amount of gravitational matter in any large volume of interstellar space.'
The data for Kelvin's investigation are as follows: The part of the universe visible to us may be considered to lie within a sphere having a radius equal to the distance of a star whose parallax is one thousandth of a second of arc. This distance is about thirty thousand million million kilometres; a distance so great that light would require about three thousand years to traverse it. The number of stars, luminous and non-luminous, within this sphere, Kelvin estimates to be something like one thousand million. This agrees well with the figures of
