6o PLEASANT WA YS IN SCIENCE. 



amount, leaving only nds. Practically, then, the advantage 

 of observing Venus, so far as distance is concerned, is the 

 same as though, instead of being at a distance of only 25 

 million miles, her distance were greater as 92 to 67, giving 

 as her effective distance when in transit some 34,300,000 

 miles. 



All the methods of observing Venus in transit are affected 

 in this respect Astronomers were not content during the 

 recent transit to use Halley's and Delisle's two time methods 

 (which may be conveniently called the duration method and 

 the epoch method), but endeavoured to determine the 

 position of Venus on the sun's face directly, both by observa- 

 tion and by photography. The heliometer was the instru- 

 ment specially used for the former purpose ; and as, in one 

 of the new methods to be presently described, this is the 

 most effective of all available instruments, a few words as to 

 its construction will not be out of place. 



The heliometer, then, is a telescope whose object-glass 

 (that is, the large glass at the end towards the object 

 observed) is divided into two halves along a diameter. 

 When these two halves are exactly together that is, in the 

 position they had before the glass was divided of course 

 they show any object to which they may be directed precisely 

 as they would have done before the glass was cut But if, 

 without separating the straight edges of the two semicircular 

 glasses, one be made to slide along the other, the images 

 formed by the two no longer coincide.* Thus, if we are 

 looking at the sun we see two overlapping discs, and by 

 continuing to turn the screw or other mechanism which 

 carries our half-circular glass past the other, the disc-images 



* The reader unfamiliar with the principles of the telescope may 

 require to be told that in the ordinary telescope each part of the object- 

 glass forms a complete image of the object examined. If, when using 

 an opera-glass (one barrel), a portion of the large glass be covered, a 

 portion of what had before been visible is concealed. But this is not 

 the case with a telescope of the ordinary construction. All that happens 

 when a portion of the object-glass is covered is that the object appears 

 in some degree less fully illuminated. 



