"Very bright, very large; oval; very gradually pretty, much brighter in the middle; a beautiful nebula; it has very much the resemblance to the Nubecula Major itself as seen with the naked eye, but it is far brighter and more impressive in its general aspect as if it were doubled in intensity. Note--July 29, 1837. I well remember this observation, it was the result of repeated comparisons between the object seen in the telescope and the actual nubecula as seen high in the sky on the meridian, and no vague estimate carelessly set down. And who can say whether in this object, magnified and analysed by telescopes infinitely superior to what we now possess, there may not exist all the complexity of detail that the nubecula itself presents to our examination?"[458] The late Lord Kelvin, in a remarkable address delivered before the Physical Science Section of the British Association at its meeting at Glasgow in 1901, considered the probable quantity of matter contained in our Visible Universe. He takes a sphere of radius represented by the distance of a star having a parallax of one-thousandth of a second (or about 3000 years' journey for light), and he supposes that uniformly distributed within this sphere there exists a mass of matter equal to 1000 million times the sun's mass. With these data he finds that a body placed originally at the surface of the sphere would in 5 million years acquire by gravitational force a velocity of about 12½ miles a second, and after 25 million of years a velocity of about 67 miles a second. As these velocities are of the same order as the observed velocities among the stars, Lord Kelvin concludes that there _is_ probably as much matter in our universe as would be represented by a thousand million suns. If we assumed a mass of ten thousand suns the velocities would be much too high. The most probable estimate of the total number of the visible stars is about 100 millions; so that if Lord Kelvin's calculations are correct we seem bound to assume that space contains a number of dark bodies. The nebulæ, however, probably contain vast masses of matter, and this may perhaps account--partially, at least--for the large amount of matter estimated by Lord Kelvin. (See Chapter on "Nebulæ.") In some notes on photographs of the Milky Way, Prof. Barnard says with reference to the great nebula near ρ Ophiuchi, "The peculiarity of this region has suggested to me the idea that the apparently small stars forming the ground work of the Milky Way here, are really very small bodies compared with our own sun"; and again, referring to the region near β Cygni, "One is specially struck with the apparent extreme smallness of the general mass of the stars in this region." Again, with reference to χ Cygni, he says, "The stars here also are remarkably uniform in size."[459]