CAMPBELL— CROCKER ECLIPSE EXPEDITION. 253 



than Professor Tucker's recent observations of the moon's position 

 had indicated, and two seconds later than the chronometer time set 

 down on the program prepared an hour before contact II. for the 

 guidance of the observers. 



The Einstein Effect. 



The search for the so-called Einstein effect has become an im- 

 portant eclipse problem. It is well known that recent hypotheses 

 of the nature of light require that rays from distant stars on their 

 way to the eclipse observer, and passing close to the sun's edge, 

 should be drawn toward the sun in appreciable amount while pass- 

 ing through the sun's gravitational field. This would cause a mi- 

 nute displacement of the stellar images upon the photographic plate. 

 Photographs of the region immediately surrounding the eclipsed sun 

 were secured by the Crocker Expedition on June 8, 1919, and pho- 

 tographs of the same region of th'e sky were obtained with the same 

 instrument set up at Mount Hamilton in January of the present 

 year. If the Einstein effect is a reality, a comparison of the two 

 sets of plates, one obtained with the sun in the field of observation, 

 and the other with the sun absent, should show slight and sys- 

 tematic differences in the angular separation of pairs of stars on 

 opposite sides of the eclipsed sun's position. Owing to war service 

 on the part of our astronomer who secured the photographs for this 

 problem at the eclipse, the plates have not yet been measured. It is 

 hoped that they will receive attention in the month of May. 



In securing both sets of Einstein photographs, the driving clock 

 should be reliable, and the observer should " guide " in right ascen- 

 sion on a bright star in the immediate neighborhood of the sun. A 

 guiding telescope of three, four, or five inches aperture and of focal 

 length equal to that of the Einstein cameras and making an appro- 

 priate angle with the axes of the cameras, should be able to pick up 

 the image of the selected bright guiding star a few seconds before 

 contact II. 



The Vulcan Problem. 



In view of Dr. Perrine's results of searches for unknown bodies 

 revolving around the sun, made under the auspices of the Crocker 



