nearest of the stars the apparent magnitude of its disc would be no greater than an eagle would seem if soaring overhead at an altitude three times as great as the distance of New Zealand beneath our feet. Of course, no instrument whatever would render the dimensions of such an object perceptible, though such is the delicacy of the sense of perception of light that the eye may be able to detect the radiation from a self-luminous object which is itself too small to form an image of recognisable dimensions on the retina. The stars, of course, are suns often comparable with, and often far exceeding, our own sun in lustre and dimensions, but their distance is far too large to enable us to measure their diameters by the ordinary processes of the observatory. Even if the stars were brought towards the earth so that their distances were reduced to a tenth of what they are at this moment, it does not seem at all likely that any one of them would be even then seen clearly enough to enable us to perceive its diameter.
This statement becomes the more significant when it is borne in mind that there are several cases in which, though we are not able to measure the dimensions of stars, yet we are able to weigh them. If the period of revolution of a binary star has been determined, and if the distance of the pair from the sun is also known, we then have sufficient data to enable us to compare the mass of the binary system with that of the sun. It will therefore be understood that the first observations which declare the actual dimensions of a star merit the utmost attention. They constitute a distinct and important departure in our knowledge of the universe. It is surely a noteworthy epoch in the history of astronomy when, for
