THE COMING TRANSIT OF VENUS. 215 



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. Win- 

 lock, 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 de- 

 pendence 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 fol- 

 lows : 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 spectro- 

 scope, 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. 



