398 MCSMX W H. M. Christie ;inl F. W. Pys.m. 



(2) Small-scale Plwtoyraphs to sliow Extension of th* 1 t',,i,,i,<i, 

 (These were taken by Mr. C. Davidson.) 



The double camera, used in former eclipses, was adapted to cany ;i 

 Dallmeyer rapid rectilinear lens of 4 inches aperture and 34 inches focus, 

 working at f/8 in one-half of the tube. This lens was lent by the Royal 

 Astronomical Society, the two halves, which had been used separately 

 in former eclipses since 1882, having been reunited for the present 

 eclipse. 



In the other half of the camera tube there was mounted a new 

 "Unar" lens by Ross, of 2'4 inches aperture and 12 inches focus, 

 working at f/5. 



Four plate-holders, each taking a pair of 16x16 cm. plates side by 

 side were used during totality, both plates being exposed by a quarter 

 turn of a shutter. A fifth plate-holder was used to obtain double 

 images of the Sun for orientation as soon as practicable after totality. 



The double camera was mounted on the equatorial stand of one of the 

 Dallmeyer photo-heliographs, originally made for the Transit of Venus 

 1874, the middle section of the stand being removed to make it more 

 handy. 



Both lenses were carefully focussed on star-fields, the final adjustment 

 being made by inserting thin metal rings teneath the flange of the 

 lens. 



The four slides for photographs of extension of the corona were 

 exposed as below, the exposures being noted by the observer, counting 

 from the commencement of totality. 



Exposure. 



Begin- Dura- 

 No, ning. End. tiun. Plate. 



1 3 s 8" 5" Sandell Double-coated. 



2 18 48 30 Triple-coated. 



3 57 72 15 



4 80 83 3 Empress. 



Shortly after totality the fifth slide with Sandell double-coated plates 

 was exposed three times on the Sun for orientation at intervals of 3i 

 mins. with the driving clock stopped. In this case both lenses were 

 stopped down to their smallest aperture, i.e., f/64 for the " Unar " 

 lens and f/44 for the Dallmeyer, and the exposure was as short as 

 possible (about -\ sec.). 



" Abney squares " were put on Nos. 2 and 4 after return home. 

 No. 2 30 sees, exposure at 5 feet to standard candle. 

 No. 4 10 



The photographs with short exposure No. 4 3 sees, are the most 



