112 NEW MEASUREMENTS OF DISTANCE OF SUN. 



photogTaphic equatorial just completed and made to work. (I may 

 remark parenthetically that it took longer to make the machine 

 work than to build it, for when one embarks upon a large experiment 

 and sets up an instrument, the first of its kind, built upon improved 

 lines, one sets out upon a sea of troubles.) 



The great advantage of the photographic method in such an under- 

 taking must be sufficiently well known l)y you. It is, of course, this, 

 that one is rendered very much more independent of continued fine 

 weather. A photograph of the planet and the surrounding stars 

 could be made in two or three minutes of actual exposure. Given an 

 hour's break in the clouds, one could accumulate far more valuable 



SCALE: SECS OF ARC. 



I I I I I I I I I 



Fig. 4.— ML-ridians and parallels of latitude of the Earth as seen from Ero.s, Dec. 2, 1900 (negative). 

 The blackened portion represents the bright crescent of the Earth, the planet being more than a 

 month past opposition. 



material than could be obtained in a whole night's visual observation, 

 for the photograph once secured could be measured at leisure, by day 

 or on cloudy nights. 



Throughout Europe the skies of that winter were far from clear. 

 I had the pleasure at Cambridge of sitting up from dusk till dawn for 

 nearly three months on end and during that time had not half a 

 dozen nights clear right through. Had I been making visual observa- 

 tions I should have done little; as things turned out I was able to get 

 some five hundred exposures. The ]n-ogramme was to get four ex- 

 l-»osures per hour throughout the night, making a number of exposures 

 on each plate, and moving the i^late a little between each ex])osure. 



