180 



Dr. A. Schuster. 



[Mar. 17, 



of the blastopore before the closing of the primitive streak region, or, 

 to this area minus the secondary extension, caused by the projecting 

 yolk, in Monotremata. 



IV. "On the Total Solar Eclipse of August 29, 1886 (Pre- 

 liminary Account)." By Arthur Schuster, Ph.D., F.K.S., 

 Professor of Applied Mathematics in Owens College, Man- 

 chester. Keceived March 3, 1887. 



The instrument entrusted to me by the Eclipse Expedition was 

 similar to that employed in Egypt during the eclipse of 1882. The 

 equatoreal stand carried three cameras, one of which was intended 

 for direct photographs of the corona, while the two others were 

 attached to spectroscopes. 



JPhotograplis of the Corona.' — The lens had an aperture of 4 inches, 

 and a focal length of 5 feet 3 inches ; giving images of the moon 

 having a diameter of about - 6 of an inch. 



During the first minute of totality the corona was covered by a 

 cloud, which was, however, sufficiently transparent to allow the 

 brightest parts of the corona to show on the two photographs exposed 

 during that time. 



During the remaining time, that is to say, during about two 

 minutes and a half, the sky was clear, but there were clouds in the 

 neighbourhood of the sun. 



The time of exposing the photographs, which had been fixed before- 

 hand, had to be altered in consequence of the uncertainty of the 

 weather, and for this reason I can only give the actual times of expo- 

 sures very approximately and from memory. One photograph on 

 sensitive paper shows only little detail; but three photographs on 

 glass were obtained, which, as regards definition, I believe to be equa] 

 to those obtained in Egypt. The approximate exposures were 15 to 

 20 seconds, 10 to 15 seconds, and about 5 seconds. 



With the view of possibly increasing the amount of detail, which it 

 lias hitherto been possible to obtain on the photographs of the corona, 

 I have, on this occasion, given considerable attention to the different 

 adjustments, so as to fix the cause which at present limits the defini- 

 tion. A telescope of 4 inches aperture ought to separate two small 

 objects which are at an angular distance of about 6 seconds of arc. 

 The theoretical resolving power can only be realised with small and 

 distinct objects like double stars ; in an object like the corona it is 

 difficult to estimate the resolving power actually obtained. Neverthe- 

 less, as far as I can judge, the photographs obtained in Egypt and in 

 Grenada with the same instrument show no detail which theoretically 

 could not have been seen with an aperture of 1 inch. The photo- 



