TEANSACTIONS OF SECTION A. 591 



belief in the existence of such a stratum has been chiefly based upon an observa- 

 tion made by hioi in Spain in 1870. The necessary funds for the expedition— the 

 spectroscopic part of it — were provided by the liberality of certain friends of the 

 •College of New Jersey (commonly known as Princeton College). At first only 

 spectroscopic observations were contemplated ; but later Professor Libbey offered 

 to accompany the expedition at his own expense, and look after the photographic 

 operations, provided suitable apparatus could be obtained. 



Pei-sonnel. — The party consisted of seven persons : Professor C. A. Young, Pro- 

 fessor M. McNeill, Professor W. Libbey, Jun., Mrs. Libbey. Miss Boyd, Miss Yeo- 

 mans, and Mr. F. Fisher, the mechanician of the party. 



Instruments. — (a) A photographic telescope, loaned by the Navy Department 

 of the United States Government. The instrument has a 6-inch lens by Dall- 

 meyer, with a focal length of about four feet. It was mounted on an equatorial 

 stand with clockwork, and was intended to give a series of pictures of the 

 corona. An ingenious apparatus had been applied by Professor Libbey for expos- 

 ing the whole series of eleven plates without the necessity of drawing any slides 

 or doing anything likely to disturb the pointing of the instrument. An ordinary 

 camera of large field was also mounted on the same stand and carried by the clock- 

 work — Professor Libbey's instrument. 



{b) A fixed photographic telescope of 6-inch diameter and 8-feet focus : the 

 lens, however, was not specially corrected for the photographic rays. It was en- 

 trusted to us by Professor W. H. Pickering, of Harvard College Observatory, to be 

 used in making a series of pictures for comparison with a second series to be made 

 by a similar instrument in Japan by Professor Todd. It had no mounting or 

 <^lockwork, but was to be simply blocked up into position and used fixed. The 

 ladies were to manipulate it. 



(c) A large direct- vision half-prism spectroscope by Hilger, with collimator of 

 about 40 inches focal length, and a prism capable of taking in 2A-inch beam. With 

 the eyepiece used the dispersion is sufficient to show I) widely double, and E 

 easily so. The slit is about an inch in length. In front of it was placed an achro- 

 matic object-glass of about 2 inches diameter and 18 inches focus, forming on the 

 slit plate a small image of the sun, about one-sixth of an inch in diameter. The 

 instrument was mounted upon a portable equatorial stand, and was in charge of 

 Professor McNeill, to be used in studying the extension of the corona line on the 

 ■east and west sides of the sun, and in examining the general structure of the corona 

 spectrum with reference to the question of the existence in it of true dark Fraun- 

 hofer lines, or bands of other sorts. 



{d) A 5-inch achromatic telescope of 6 feet focal length, equatorially mounted, 

 and provided with a gi-ating spectroscope. Telescope and collimator of the spectro- 

 scope have each a diameter of about li inch, and a focal length of 14. The 

 grating, by Rutherfurd, has 17,280 lines to the inch, the ruled space being 1 f inch 

 by 2^ nearly. 



In the eyepiece was placed a scale made by photographing a small portion of 

 the map of the spectrum just below F, including certain groups of lines which Mr. 

 Lockyer has pointed out as specially adapted to throw light on the questions in- 

 volved by their behaviour at the beginning and end of totality. 



The instrument also had attached to it a small integrating spectroscope of one- 

 prism dis])ersion, with which the general corona spectrum could be observed at the 

 middle of totality. 



It was not thought best under the circumstances to attempt any spectrum- 

 photography, as we expected that to be provided for by European parties. 



Station, Sfc. — The station selected for us was a country house about eight miles 

 north-east of the town of Rschew, a city of some 30,000 inhabitants, at present the 

 terminus of a railway which branches off at Ostaschkowo from the main line be- 

 tween Petersburg and Moscow. Our lat. was 56° 22' ; long. 16m. 04s. east of Pul- 

 kowa. The station was selected and all arrangements made for us by Dr. Struve, 

 the director of the Pulkowa Observatory, to whom we are indebted to an extent 

 not easily to be expressed in words. Our instruments were passed through the 

 Custom House free of duty and without examination, and were forwarded to our 



