On the " Black Drop " in the Transit of Venus. 375 



forward, through the rise of the isogeotherms, and that "this in an 

 important degree has made possible the catastrophe from which 

 synclinoria have resulted," is attributed (p. 374) to Professor 

 LeConte. I should have credited to Professor T. Sterry Hunt the 

 idea of the weakening of the bottom of a geosynclinal in the man- 

 ner stated. To this idea Professor LeConte added the view that, 

 through such a weakening, lateral pressure from the earth's 

 contraction (a force not appealed to in Professor Hunt's hypo- 

 thesis) was enabled to produce the catastrophe referred to ; and 

 this is the important principle adopted from his memoir. — J.D.D.] 



XL VII. On the "Black Drop " in the Transit of Venus. 

 By T. K. Abbott, M.A., Fellow of Trinity College, Dublin*. 



THE recent discussion in the British Association respecting 

 the " black drop " observed in the transit of Venus, in- 

 duces me to send you an account of a simple experiment which 

 furnishes a very striking illustration of that phenomenon, and 

 may be considered as affording an experimental proof that the 

 moment of formation of the " drop " is the moment of optical 

 contact. A common penknife and a threepenny piece (the 

 thinner the better), or any two even edges, provided one is 

 curved, constitute a sufficient apparatus. Bring the two edges 

 into contact and hold them between the eye and a tolerably 

 bright light, such as the flame of a candle, or better a lamp- 

 globe, or even a window. At the moment of actual contact the 

 edges appear to be, not touching, but joined by a short liga- 

 ment, precisely as in the case of the transit. It is necessary 

 that the eye be so adjusted that the point of contact shall be the 

 point of most distinct vision ; otherwise the ligament will appear 

 longer but less distinct, and crossed by one or more short dark 

 lines. If a coin with a milled edge be used, or a torn edge of 

 paper &c, an appearance like Baily's beads is presented. It 

 is easy to arrange the objects in such a manner that they can be 

 fixed at any required distance from each other short of actual 

 contact; and it will be found that the ligament is not formed 

 until contact (actual or optical) occurs. Indeed it is only ne- 

 cessary to bring the fore finger and thumb together and hold 

 them up to the light in order to observe the phenomenon ; but 

 it is less easy in this case to determine when optical contact oc- 

 curs. It is, however, easier to " fix " the eye on the fingers than 

 on the dark edges first mentioned. In each instance it is curious 

 to observe the apparent adhesion of the ligament. An excellent 

 example of the phenomenon may be produced by touching the 

 edge or back of the knife with the point of a pin, holding the 



* Communicated by the Author. 



