OF ARTS AND SCIENCES. 21 



become evident that the cusps were separated by a part of the planet's 

 limb, and that geometrical contact was past. 



Tiiese observations were made through a dark red shade-glass be- 

 tween the eyepiece and the eyestop. The limb of the sun was dis- 

 tinctly seen, and was free from glare. As the sun entered the thin 

 clouds mentioned in the introductory remarks above, the red glass 

 was replaced by a blue one, which admitted much more light. The 

 part of the planet exterior to the limb of the sun was then certainly, 

 though indistinctly, seen. Its outline seemed to be part of a smaller 

 circle than that bounding the portion of the disk interior to the sun's 

 limb. The increasing cloudiness soon put an end to this appear- 

 ance, the Greenwich mean time of which, derived from the record, 

 23 7" 57™ 3 P. 



The fourth contact was observed with some difficulty, owing to the 

 clouds and to the necessity of an occasional movement of the mirror 

 to keep the image in the field. The time given is that of a signal 

 accompanied by the remark " Notch doubtful " ; the notch was not 

 afterwards seen. 



During the transit, the disk of the planet was uniformly dark, ex- 

 cept that at times it seemed to be crossed by faint streaks of light, 

 very likely due to slight defects in the shade-glass or other parts of 

 the optical apparatus employed. 



The eyepiece used was positive, No. 5 of the set of eyepieces be- 

 longing to the large filar micrometer of the East Equatorial, and, 

 with that instrument, having a nominal magnifying power of 688. 

 Its power, with the telescope used during the transit, has been deter- 

 mined by two methods, and the approximate mean result 220 is given 

 in the Table. 



The chronometer employed was Bond 236, regulated to sidereal 

 time ; it has been in constant use at the Observatory for many 

 years. 



0. a Wendell. 



The instrument employed was the finder of the East Equatorial. 

 The object-glass was silvered to reduce the light, and an additional 

 reduction was effected by a shade-glass. Between the first and second 

 contacts the silvering was partially removed, owing to an apprehension 

 that the clouds would grow thicker ; but as the sky actually became 

 clearer, the second contact could not be observed. Before the third con- 

 tact, the film of silver was entirely removed, and the object-glass was 

 smoked by Mr. Clacey's method, which sufficed, with the aid of one 

 shade-glass, to reduce the light. 



