OF ARTS AND SCIENCES. 19 



observer, obtained by adding 4'' 44™ 3P to each of the Cambridge 

 mean times. The mean result for each contact is given in the last line 

 of the Table. At the right are given the differences of each observer's 

 result from the mean of all. 



The following notes contain, under the name of each observer, the 

 details of his work. 



E. C. Pickering. 



The instrument employed was the East Equatorial. Its full aper- 

 ture is 1 5 inches, which on this occasion was reduced to about 6 inches 

 by a cap over the object-glass. An audible signal was given to the 

 recorder at the time of each phenomenon noted. The recorder took 

 the time of each signal from the chronometer, and recorded it, with 

 any subsequent remarks by ihe observer. The wedge of shade glass 

 placed between the eyepiece and the eye was of a greenish tint. In 

 observing the first contact, the last time recorded before the appear- 

 ance of the notch was ^^ 19"" 29^6 by the chronometer. Venus was 

 first seen at ^^ 19" 44^2 by the chronometer. The edge of the sun 

 was wavy, rendering it difficult to decide whether an indentation was 

 real. At 9^ 19™ 49^0 the interval between the cusps was estimated 

 at 9" ; two parallel lines 6" apart served as the unit of measure. 

 From a reduction of this observation the time of first contact appears 

 to be 9*^ 19™ 42'. 6 ; the mean of this and of the time directly observed 

 is here assumed to be the time of the first contact, which is therefore 

 gh jQm 4334 jjy ^}jg chronometer. A scale in the eyepiece would allow 

 the observer of phenomena like these to make estimates of the inter- 

 val of the cusps without removing his eye from the telescope, and 

 would accordingly afford him many of the advantages of a double-image 

 micrometer without its disadvantages. 



The images at the second contact were unusually well defined, and 

 the contact was recorded as occurring at 9^ 39™ 5r.3. Eight seconds 

 later it was clearly past. 



The third contact was recorded as occurring at 3^ 3™ 3'.8. At 

 gh 3m 2is_4 tijg interval between the cusps was estimated as double 

 that between the lines in the field, and consequently as 12". A 

 reduction of this observation would make the time of contact 

 3'' 3™ 0^0. 



The fourth contact was recorded as occurring at 3'' 23™ lO'.O. 

 At 3*^ 23™ 3^4 it had not occurred, at 3'^ 23™ 19^4 it was certainly 

 past. 



The chronometer used by the recorder, who also recorded for Mr. 



