Continental astronomers have another way of doing this which we will attempt to explain. Suppose we wish to find the difference in declination of a star and Jupiter, we place the ring, A D, Fig. 163, in the eyepiece of the telescope and watch the passage of Jupiter and the star over this ring micrometer. It will be clear that, as the motion of the heavens is perfectly uniform, it will take very much less time for the star to travel over the ring from B to C than it will for Jupiter to travel over the ring from _b_ to _c_, because the star is further from the centre; and by taking the time of external and internal contact at each side of the ring, the details of which we need not enter upon here, the Continental astronomers are in the habit of making differential observations of the minutest accuracy by means of this ring micrometer, whilst we prefer to make them by the wire micrometer. BOOK VI. _ASTRONOMICAL PHYSICS._ CHAPTER XXV. THE GENERAL FIELD OF PHYSICAL INQUIRY. We have now gone down the stream of time, from Hipparchus to our own days. We find now enormous telescopes which enable us to see and examine celestial bodies lying at distances so great that the mention of them conveys little to the mind. We find also perfect systems of determining their places. The following chapters will show, however, that modern astronomy has not been contented with annexing those two branches of physics which have enabled us to make the object-glass and the clock, and another still which enables us to make that clock record its own time with accuracy. These applications of Science have been effected for the purpose either of determining with accuracy the motion and positions of the heavenly bodies or of enabling us to investigate their appearances under the best possible conditions. The other class of observations to which we have now to refer, have to do with the quantity and the quality of the vibrations which these bodies impart to the ether, by virtue of which vibrations they are visible to us. We began by measurement of angles, we end with a wide range of instruments illustrating the application of almost every branch of physical as well as of mathematical science. In modern observatories applications of the laws of Optics, Heat, Chemistry and Electricity, are met with at every turn.