Page:Popular Science Monthly Volume 4.djvu/227

Rh at leisure, and without the hurry and anxiety which attach to any observation of this nature.

This method requires apparatus of a special kind. The American plan is to throw the image of the sun, with the planet on its disk, into a stationary photographic telescope where the negative is taken. This is taken out, and at once developed by the photographers, into whose dark room the telescope penetrates. This method is due to Prof. Winlock, of Harvard College Observatory. The other method consists in making the photographic telescope follow the sun in its motion by means of clock-work, and in taking the negatives in the same way. The dark room, however, is some distance off, and it appears that too much dependence must be placed on the steadiness of the clock-work motion.

4. The heliometric method. This consists in measuring the cusps with a heliometer, which is merely a large telescope which has two object-glasses (or one object-glass cut into halves by a diametral cut) which slide past each other. Each half produces a complete image, and, by means of an observation of a tangency of images, the distance of the cusps may be had.

5. The spectroscopic method. In brief, we may explain this as follows: It is known that there is a thin layer of atmosphere near the sun's limb where bright lines may be seen with a powerful spectroscope, while on either side of this layer dark lines only are seen. As Venus advances, the interposition of her dark body will cut off this layer, and the instant of disappearance of the last vestige of any one of these bright lines will be truly the instant of first contact.

The ordinary method of observing first contact is open to grave uncertainties (on account of the different sensitiveness of the eyes of various observers, and for other reasons), and it is hoped that this method, as beautiful in theory as it will be difficult and delicate in practice, will obviate all these objections.

It is to be expected that the astronomers of the different nations will adopt different plans of observations, in accordance with the peculiar traditions of each school.

The Germans and Russians, among whom the use of heliometers has been hitherto confined, will (with a single exception) alone use them on the approaching transit.

The German Government will send one of these instruments to the Kerguelen Islands, or to Macdonald Island, one to the Auckland Islands, one to the Mauritius, and one to China (Chefoo). Lord Lindsay, of England (the one exception spoken of), also takes a heliometer with his very completely-equipped private expedition to the Mauritius.

Three of the twenty-seven Russian stations in Russia, Siberia, China, and Japan, will be provided with heliometers; at three, like-wise, will the photo-heliograph be used, while the remainder of the stations will be devoted to the ordinary contact observations and to measures of cusps.