NEW WA YS OF MEASURING THE SUN'S DISTANCE. 61 



of the sun may be brought entirely clear of each other. 

 Then we have two suns in the same field of view, seemingly 

 in contact, or nearly so. Now, if we have some means of 

 determining how far the movable half-glass has been carried 

 past the other to bring the two discs into apparently exact 

 contact, we have, in point of fact, a measure of the sun's 

 apparent diameter. We can improve this estimate by carry- 

 ing back the movable glass till the images coincide again, 

 then further back till they separate the other way and finally 

 are brought into contact on that side. The entire range, 

 from contact on one side to contact on the other side, gives 

 twice the entire angular span of the sun's diameter ; and the 

 half of this is more likely to be the true measure of the 

 diameter, than the range from coincident images to contact 

 either way, simply because instrumental errors are likely to 

 be more evenly distributed over the double motion than over 

 the movement on either side of the central position. The 

 heliometer derived its name which signifies sun-measurer 

 from this particular application of the instrument 



It is easily seen how the heliometer was made available 

 in determining the position of Venus at any instant during 

 transit The observer could note what displacement of the 

 two half-glasses was necessary to bring the black disc of 

 Venus on one image of the sun to the edge of the other 

 image, first touching on the inside and then on the outside. 

 Then, reversing the motion, he could carry her disc to the 

 opposite edge of the other image of the sun, first touching 

 on the inside and then on the outside. Lord Lindsay's 

 private expedition one of the most munificent and also one 

 of the most laborious contributions to astronomy ever made 

 was the only English expedition which employed the 

 heliometer, none of our public observatories possessing such 

 an instrument, and official astronomers being unwilling to ask 

 Government to provide instruments so costly. The Germans, 

 however, and the Russians employed the heliometer very 

 effectively. 



Next in order of proximity, for the employment of the 



