Mayer.] -^"^ [Oct. i, 



AN ABSTRACT OF SOME OP TUB RESULTS OF MEASUREMENTS AND 

 EXAMINATIONS OF THR PHOTOGRAPHS OF THE TOTAL SOLAR 

 ECLIPSE OF AUGUST 7, 1869. 



By Alfred Mayer, Ph. D. 



Professor of Physics and Astronomy in the Lehigh University, Penna. 



I have the honor to lay before the American Philosophical Society a 

 few results of my measurements and examinations of the Photographs 

 of the Total Solar Eclipse of Aug. 7, 18G9, taken at Burlington, Iowa. 



The Photographic expedition, of which these photographs are part of 

 its results, was organized by Prof. Henry Morton of Philadelphia, under 

 the authority of Prof. J. H. C Coffin, U. 8. K"., Superintendent of the 

 American JSTautical Almanac. 



The expedition occupied three stations in Iowa; viz. Burlington, Mt. 

 I'leasant and Ottumwa, respectively under the charge of Professors 

 Mayer, Morton and Himes. 



Burlington is situate in Lat. N. 40° 48' 21. "58; Long. Oh. 56m. 13.88s. 

 W. of Washington. It was 7 miles N. of the centre of the moon's shadow. 



The telescope used was by Merz & Mahler of Munich, and is the 

 proiDerty of the Central High School of Philadelphia. It is eciuatorially 

 mounted; of 6.42 inches aperture and of 9 feet focus, and is driven by 

 one of Fraunhofer's fi-iction governor clocks. 



The sun's image was formed on the plate of the camera by a Huy- 

 ghenian eye-piece, the lenses of which were specially computed to give 

 the least aberration when they formed an image of the sun in the camera 

 of 2.04 inches diameter. The image of a reticule of two spider threads 

 at right angles to each other was also projected on the plate with the 

 sun's image, and tliese threads were by me accurately adjusted, the one 

 parallel, the other at right angles to the celestial equator; thus, the pho- 

 tographs have given precise ]30sition-angies of the contacts and of the 

 protuberances. 



A plate having a transverse slot of ^^gth of an inch in breadth shot 

 across the eye-piece by the action of a spring and thus gave the exposure 

 during partial phase. The duration of this fMsh of the sun upon the 

 camera has, since the eclipse, been made by me the object of an experi- 

 mental determination, and by means of an electrical break-circuit clock 

 and chronograph, I have found that the exposure of the collodion plate 

 to the sun's image was almost exactly the 5^0 ^^^ o^ ^ second. 



[Prof. Mayer here exhibited the camera used; explained? the action of 

 the exposing plate and how by its fall the time of exposure was electri- 

 cally recorded on a chronograph fillet; and showed the arrangement of 

 the aparatus by which he determined the time of exposure.] ' 



While taking the photographs during partial-phase only 2 inches aper- 

 ture of object-glass was used; but, during totality the full aperture was 

 employed and the slide-plate allowed the whole beam to fall upon the 

 plate. The exposure of the totality photographs varied from 5s. to 7s. 



Forty-one perfect photographs were taken during the eclipse, and five 

 of these were obtained during totality, which lasted 2m. 42s. The five 

 totality pictures were taken in one minute and thirty-nine seconds. 



