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THE POPULAR SCIENCE MONTHLY. 



sible time for her disk to move from first contact (when her disk just 

 touches the disk of the sun exteriorly) to second contact (when her 

 disk is tangent interiorly to the sun's), and during this time the ap- 

 pearance of the two disks will be as in the figure : 



This figure shows Venus coming on to the sun's disk, and it shows 

 the two cusps at a and b. It will be easily seen that, if we know the 

 length of the line a 5, and the time at which it has this length, we 

 can calculate the time of contact from these data. So that a number 

 of measures of the cusps is the same as a number of first contacts. The 

 reverse phenomenon occurs when Venus passes off the sun's disk. 



To measure these distances, the equatorials must be provided with 

 filar micrometers, i. e., with a contrivance by means of which two 

 spider-lines in the focus of the telescope may be moved toward or 

 away from each other. One of these lines is to be placed at a, and 

 the other at b / the time is to be noted, and the number of turns and 

 parts of a turn of the screw which moves the lines is to be noted from 

 the head of the screw, which is finely divided. 



3. The photographic method. This consists in photographing the 

 planet Venus on the disk of the sun, and noting the time of each pho- 

 tograph. The negatives are carefully preserved, and are measured 

 subsequently by a fine measuring engine. It will be seen that this 

 method is like the preceding, except that the measuring may be done 



