82 LOCAL PAETICULARS OF THE TRANSIT OF VBKUS, 1874. 



Gilt, and two flat edges cut to it (figure 10), wliicli represents 

 ttie lens so cut, then the sockets for the lens must be cut to 

 the same curve as the lens, and lined with soft leather ; two flat 

 pieces must then be fixed in the eye-piece bo as to touch the 

 flat edges of the glass and keep it from turning round ; lastly, a 

 spring must be placed, tending always to press in one part of the 

 lens, and a screw to press in the other part, by which motion 

 in or out may be given to that end of the lens. In use, then, if 

 the atmospheric dispersion become troublesome, it is only necessary 

 to turn on the screw until the prism so formed is equal to the 

 effect required. (The following notes are based on Mr. Stone's 

 paper in Eoyal Astronomical Society's Notices.) 



In figure 7 I have represented "Venus in the several stages of 

 ingress and egress, and have shown the apparent edge of the sun 

 by the continuous line and the real edge by the dotted line. I 

 have on purpose grossly exaggerated the diff"erence between these, 

 in order to make the explanation more obvious, and I have more- 

 over assumed that the whole amount is on the sun, for simplicity ; 

 but in fact the dark body of Venus is reduced when in transit 

 by the same amount, and from just the same cause as the sun's 

 is increased. Starting then with No. 1, in which Venus is 

 bisected by the sun's apparent edge, we notice that it is not 

 really bisected, and that it will not be bisected until the centre 

 of Venus comes on the dotted line, as iu No. 2. Now observe 

 what takes place here. All the light between the continuous and 

 dotted lines is fictitious ; and, since the real light is cut off" at the 

 dotted line, Venus takes the form represented, that of a dome 

 resting on the sun's edge. In No. 3, the light cut off" at the 

 dotted line is not equal to the diameter of Venus ; and it appears 

 as if a portion of the edge of the planet were drawn out to the 

 apparent edge of the sun. This is called the formation of the 

 drop. In No. 4 the drop is smaller ; and it appears to the 

 observer that if Venus were only round, its outline would be 

 within that of the sun — that is, apparent internal contact ; but it 

 is only apparent, for it will be seen that a part of Venus still 

 covers a fraction of the sun's limb, hence a drop still remains. 

 This is one of the two phenomena that we are particularly 

 requested to observe with the greatest care, because, owing to the 

 uncertainties of atmosphere, it is very doubtful whether observers 

 will be able to see the real contact satisfactorily. The use of the 

 micrometer may be of great assistance here, as I have before 

 pointed out, for it will enable the observer to estimate more 

 accurately when Venus, if made round, will just meet the 

 apparent edge of the sun. Card 2 should be used for represent- 

 ing, at this stage, the diameter of the drop ; it should be estimated 

 as a part of the diameter of the planet, and full notes written on 

 the card, in reference to the observation at the time, expressing 



