1889.] KEW YOKK ACADEMY OF SCIENCES. 49 



The Eoyal Astronomical Society' has issued ''stringent in- 

 structions" to two observers, each with an assistant, to make 

 the observations necessary to answer the following questions: — 

 (1) Whether the photographed corona changes perceptibly in 

 two hours and a half. (2) Whether the supposed "extension " 

 of the corona is a solar appendage or diffused light from the sky. 

 (3) What is the photometric intensity of the corona at different 

 points ? The observers chosen are Rev. S. J. Perry, F.E.S., 

 who, with his assistant, Mr. Rooney, will go to the Salut Islands, 

 and Mr. Albert Taylor and assistant, who are to be stationed on 

 the coast of Africa. Thus the observers will be many thousands 

 of miles from each other. They will be equipped with precisely 

 similar instruments. They will expose a few plates at times 

 arranged upon, and on comparing the plates it will be seen 

 whether the corona has changed photographically in two hours 

 and a lialf. Question (2), as to whether the supposed ''exten- 

 sion " of the corona is a solar appendage or diffused light from 

 the sky, they will attempt to answer also by taking photographs. 

 For this purpose the observers must have telescopes which will 

 give enough light to enable the delicate "extensions" to be 

 developed. Refractors with large apertures and lequired short 

 focus are difficult to obtain for eclipse Avork; so that the English 

 parties are supplied with reflectors made by Mr. Common. The 

 mirrors are 20 inches aperture, 45 inches focal length. These 

 mirrors photographed a great portion of the very faint nebula in 

 the Pleiades in 30m., and the earth-lit portion of the moon in 2m. 

 to 3m. If the photographs at the two stations agree in what 

 they show of the extensions, then it would prove the extensions 

 to be solar, and not atmospheric. If due to the atmosphere, the 

 extensions would not be similar as seen in the two places. 



The English parties will also take with them twin refracting 

 telescopes (4 inch aperture, 5 feet focus), arranged for photo- 

 graphic work. Thus at each station will be a mirror and a lens, 

 — an instrument for each observer and his assistant, — and each 

 man takes three or four photographs. The comparisons of the 

 two sets of plates ought to settle questions (1) and (2). 



The third question will be answered by " placing beforehand 

 on each plate a set of standard photographic squares, such as 

 were described by Captain Abney to the Photograpliic Society, 

 England, in 1885, and have since been used by W. H. Pickering 

 at Grenada in 1887, an J by him and others in California on 

 January 1st of this year." 



No spectroscopic work has been undertaken, for fear of en- 

 dangering the success of the other observations. 



' The Observatory, November, 1889, p. ;196. 



