TEiLNSIT OF VENUS, 103 



would be seen when a fine line of light was emerging from behind 

 an atmosphere ; and that the planet should look perfectly sharp 

 immediately the sun-light appeared is also quite in accordance 

 with the supposition we have made. On the other hand, Mr. 

 Vessey, who had a good telescope in the best atmosphere, cer- 

 tainly saw nothing of this haziness at egress, and did not see 

 much of it at ingress. 



Of the drop phenomena which we all expected to see we have 

 two particularly interesting accounts, which I will quote. The 

 first is that by Mr. Hirst. Only saying that he was thoroughly 

 acquainted with the phenomenon as described by Mr. Stone and 

 others, and that he had some previous practice with the ai'tificial 

 transit, though the w^ork which was specially his, and of which he 

 had made himself master, was the management of the photo- 

 heliograph during the taking of the Janssen pictures. 



" Attached to the tube of the heliograph was a finder, consisting 

 of a single lens, 1\ inch aperture and about 4 feet focal length. 

 This was originally arranged by the maker so as to throw the 

 sun's image on to a piece of parchment fixed at its focus ; but in 

 order to adapt it to circumstances which required that one end 

 of the heliograph should be in the photographer's dark room, the 

 lens was inserted in the end of a brass tube, an eye-piece being 

 provided in the shape of a Huyghenian combination, giving a 

 power of about fifty diameters. The chromatic and spherical 

 aberration of the single lens was in a great measure compensated 

 by its extreme focal length, so that fair definition could be obtained 

 of the edge of the sun, and the existence of even minute solar 

 spots made plainly visible." 



"To diminish the light in the finder I used a thick piece of 

 orange-coloured glass, which gave an agreeable image of the sun. 

 This was placed outside the eye-lens of the eye-piece." 



" I had prepared and placed a plate in the Janssen apparatus, 

 when, on taking my usual glance at the finder, I observed the 

 disc of Venus appearing, as it were, rather more than one-third 

 her own diameter within the sun, and connected with the limb by 

 a narrow line intensely black, with an ill-defined edge. The 

 annexed diagram No. 2 represents the appearance as faithfully 

 as I can recollect ; this was about five seconds before No. 5 

 Janssen plate was begun. I had not time for more than a glance, 

 for I wished to procure a photograph of w^hat I supposed to be 

 the black drop, so universally observed by astronomers, more 

 than a century ago, at the last transit. On getting the plate 

 through, however, it showed nothing of what I had so distinctly 

 observed a few seconds before." 



" Eeferring to the finder, Venus appeared well inside the sun, 

 but apparently nearer the limb than she seemed before. The drop 

 was gone. I thought at the time that it might have broken before 



