DETERMINATION OF DEFLECTION OF LIGHT BY THE SUN'S GRAVITATIONAL FIELD. 301 



15. On June 7, having completed the development, we left Sobral for Fortaleza, 

 returning on July 9 for the purpose of securing comparison plates of the eclipse field. 



Before our departure we dismounted the mirrors and driving clocks which were brought 

 into the house to avoid the exposure to dust. The telescopes and coelostats were left 

 in situ. Before removing the mirrors we marked their positions in their cells so that 

 they could be replaced in exactly the same position. 



After our return to Sobral the mirrors and clocks were remounted ; the photography 

 of the eclipse field was commenced on the morning of July 11 (civil). The difficulty of 

 finding the field with the coelostats was overcome by making a rough hour circle on the 

 heads of the coelostats out of millimetre paper. 



The following is the list of exposures made on the field for comparison with the eclipse 

 photographs : 



The reference numbers follow the civil dates. 

 * The 4-inch plate, No. 18i, was taken through the glass (see 17, infra) to facilitate the measurement, 

 and is referred to as the scale plate. 



2 T 2 



