DISTANCE AND DIMENSIONS OF THE SUN. 409. 



amount which varies with the size of the telescope, the perfection of 

 its lenses, and the tint and brightness of the sun's image. The edge 

 of the planet's image is also rendered slightly hazy and indistinct. 



The planet's atmosphere also causes its disk to be surrounded by a 

 narrow ring of light, which becomes visible long before the planet 

 touches the sun, and at the moment of internal contact produces an 

 appearance of which the accompanying figure is intended to give an 



Fig. 4. 



idea though on an exaggerated scale. The planet moves so slowly as. 

 to occupy more than twenty minutes in crossing the sun's limb ; so 

 that, even if the planet's edge were perfectly sharp and definite, and the 

 sun's limb undistorted, it would be very difficult to determine the pre- 

 cise second at which contact occurs ; but as things are, observers, w r ith 

 precisely similar telescopes, and side by side, often differ from each 

 other five or six seconds ; and where the telescopes are not similar the 

 differences and uncertainties are much greater. The contact observa- 

 tions of the last transit in 1874 do not appear to be much more accord- 

 ant than those of 1769, notwithstanding the great improvement in 

 telescopes ; and astronomers at present are pretty much agreed that 

 such observations can be of little value in removing the remaining 

 uncertainty of the solar parallax, and are disposed to put more reli- 

 ance upon the micrometric and photographic methods, which are free 

 from these peculiar difficulties, though of course beset with others; 

 which, however, it is hoped will prove less formidable. 



The micrometric method requires the use of a peculiar instrument 

 known as the heliometer, an instrument common only in Germany, 

 and requiring much skill and practice in its use in order to obtain 

 with it accurate measures. At the late transit a single English party, 

 two or three of the Russian parties, and all five of the German, were 

 equipped with these instruments, and at some of the stations exten- 

 sive series of measures were made. None of the results, however, have 

 appeared as yet, so that it is impossible to say how greatly, if at all, 

 this method will have the advantage in precision over the contact 

 observations. 



The Americans and French placed their main reliance upon the 

 photographic method, while the English and Germans also provided 



