350 



SCmNCE. 



[N. S. Vol. VIII. No. 194. 



By hiding tlie sun's image by an occulting 

 disc in the first sweep, and then making a 

 second similar but more rapid sweep with 

 the sun's image uncovered, the sun itself 

 with all its faculse, spots, etc., is impressed 

 in the blank space left for it, and a complete 

 picture of the sun and all its surroundings, 

 with the exception of the corona, is secured. 

 This is the method employed by Professor 

 Hale. These pictures, however, show only 

 those features of the sun which are due to 

 hydrogen or calcium, and the solar surface 

 thus appears very different from the tele- 

 scopic view of it. The calcium regions 

 come out with extraordinary distinctness, 

 so much so, indeed, as at times to obliterate 

 completely the sun spots, which at that 

 moment are so distinct to the ej^e with the 

 same telescope. Admirable work of this 

 kind has also been done by M. Deslandres, 

 of the Paris Observatory, who has devised 

 an instrument similar to that of Professor 

 Hale. 



From the first photograph of a star by 

 Bond in 1850 to the present time, stellar 

 photography has gradually risen to a prom- 

 inence as remarkable as it is important. 

 The real increase of importance in this 

 work, however, has occurred within the 

 past ten or fifteen years, since the success- 

 ful introduction of the very rapid dry plate. 

 The wet, or collodion process was poorly 

 adapted to the photography of the stars, 

 and of no use whatever for comets and 

 nebulae. As implied by the term ' wet 

 process,' the plate must remain wet through 

 the entire work from its first coating with 

 collodion until its final washing as a nega- 

 tive. The exposiire time must, therefore, be 

 very limited. !N"ot only was the exposure 

 of short duration (from fifteen to twenty 

 minutes), but the plate was very slow in its 

 action compared with the dry plate of to- 

 day. The combination of these two difii- 

 culties made it impossible to photograph 

 anything except the brighter stars. Dr. 



Gould at Cordoba managed to increase the 

 exposure time by keeping a stream of water 

 playing over the plate. This, however, 

 might cause a deterioration of the film. 

 With the inherent difBculties of the wet 

 plate to contend with, it is little wonder 

 that no faint stars, nebulae or comets were 

 photographed. ^Notwithstanding this, the 

 photographs of the star clusters, etc., of the 

 southern skies obtained, under the direction 

 of Gould with an 11-inch photographic re- 

 fractor by the wet process, were of the 

 highest value, and showed, upon measure- 

 ment, a striking agreement in accuracy with 

 visual work. The same can be said of Mr. 

 Rutherfurd's photographs of the Pleiades, 

 Prsesepe, etc., which were made prior to 

 Dr. Gould's, and which were the first pho- 

 tographs of this kind. These extremely 

 valuable photographs of Eutherfurd are 

 now receiving a most thorough measure- 

 ment under the careful supervision of Pro- 

 fessor J. K. Rees, of Columbia College, 

 where Mr. Rutherfurd's negatives are 

 stored. To this institution Mr. Rutherfurd 

 left his telescope and measuring instru- 

 ments. 



In 1857 Bond had shown, by measure- 

 ment of a series of photographs of the dou- 

 ble star Mizar, that the highest confidence 

 could be placed in measures of star plates. 

 This has been fully verified in late years 

 by Gill, Elkin and others. Dr. Elkin 

 showed, in 1889, that measures of a photo- 

 graph of the Pleiades taken by Mr. Burn- 

 ham, with the great telescope at Mt. Hamil- 

 ton, had equal value with his heliometer 

 measures of the same stars. 



From the necessary conditions the collo- 

 dion process could make no advance in 

 stellar photography. Previous to 1876 ex- 

 periments had been made to get a worka- 

 ble dry plate, and for the next six years 

 more or less success had been attained in 

 their manufacture. But the photographers 

 themselves took hold of these plates with 



