The spectroscope comes in also to aid in determining the contacts with the utmost precision. The light of the solar photosphere, or body of the sun, when made to pass through the prisms of a spectroscope, spreads into a continuous band of various colors, and crossed by many faint, dark lines. Other bodies, raised to a certain heat, and emitting light, give a spectrum of a totally different character. We see only bright upright lines. There is no continuous band or spectrum of prismatic colors. Now, just outside the solar photosphere, and between it and the chromosphere, is a layer of solar atmosphere which gives just such upright, bright lines. This was first discovered not many years ago during a total solar eclipse, when the direct light of the photosphere was cut off by the interposing moon. Knowing what to look for, the astronomers have since been able so to manipulate their telescopes as to catch these bright lines, even when there is no eclipse. They find them, of course, as they examine, a narrow ring apparently encircling the sun, and immediately around his circumference. Now, when the moment of the beginning of the transit is at hand, the spectroscope is turned to the precise point where Venus will touch the sun’s rim, and these lines are clearly brought into vision. So long as they shine, the way is open for the light of that narrow layer or belt to reach the earth. The instant their bright flash disappears, the observer knows that the planet has so moved as to intercept the rays of light, and is just in contact. Their reappearance, at the proper time, on the other side of the sun, will indicate the instant when Venus will have quitted the disk and the transit is over. It is confidently expected that by some one or by all of these methods the uncertainties of 1761 and 1769 will be avoided, and that the instants of the commencement and the conclusion of each line of the transit may be so accurately determined that for neither of them will the error as to their duration exceed one second. Did the time occupied by Venus in making the transit, as seen from one station, differ from the time as seen at the other by only one minute, the uncertainty of one second would be less than two per cent. But, in fact, the times will differ by fifteen minutes, and, by skilfully choosing the places, a difference of twenty minutes may be obtained. In that case, the error or uncertainty would be less than one‐ tenth of one per cent. For the present, the scientific world will be satisfied with that degree of exactness.