_Theophilus._ It is then possible that our astronomers are now observing stars which no longer exist, and are only visible to us because the light which they omitted is still travelling towards the earth. _The Author._ Just so. But to continue. I have begun with the stars which are nearest to the earth. There are the stars of first and second magnitude. You know, I suppose, the signification of those terms first, second, and third magnitude in astronomy? _Theophilus._ Yes, I know that the word magnitude is only applied to the luminous appearance of the star, and not to its real bulk. A star of the first magnitude is one which forms part of the group of the most luminous stars; a star of the second magnitude is one which comes next in point of brilliancy. _The Author._ You must bear in mind that the word _magnitude_ signifies in astronomy the opposite of that which it expresses. The more luminous a star appears to us, the nearer it is to us; the paler and less visible, the farther it is away. The brilliance diminishes in proportion as the figure increases. This is an introversion of terms, sufficiently exceptional to be taken note of, and it ought to be remembered, for fear of mistakes. Hitherto we have considered only stars of the first and second magnitudes. Those of the third, fourth, fifth, and sixth, lead us to the contemplation of such immense distances, that the unit which we have adopted, enormous as it is, is no longer of use. The instruments of celestial observation which may be applied to the examination and measurement of stars of the first and second magnitudes, do not serve for stars of the third and following magnitudes, and, because the small visible diameter of those stars make them appear mere specks of light, measuring instruments are equally inapplicable to them. In estimating the distances of the stars after the third magnitude, a method of comparison, based on the amplifying power of the telescopes successively used, is employed. I cannot enter into details of this method, which we owe to Sir William Herschel, but must content myself with explaining its results, which are as follows in the case of stars of the sixth magnitude. From certain stars of that class, light would take 1042 years to reach us: from others it would take 2700. After the sixth magnitude, the stars can only be discerned by the aid of the telescope, and their distances become perfectly stupefying in immensity. Certain of these telescopic stars are so far from the earth, that their light can only reach us in 5000, and even 10,000 years after it leaves the luminous centre. From the stars of the last category (fourteenth magnitude), light would take 100,000 years to reach the earth, supposing it to travel at the same rate as the light of our sun, _i.e._, 77,000 leagues per second.