1886.] on Photography as an Aid to Astronomy. 373 



the nearest pair it has been tried ujoon. This distance is quite 

 manageable provided the stars are of nearly equal magnitude. 



" The power to obtain images of the ninth magnitude stars with so 

 moderate an aperture promises to develop and increase the application 

 of ijhotography to the mapping of the sidereal heavens and in some 

 measure to realise the hopes which have so long been deferred and 

 disappointed. 



" It would not be difficult to arrange a camera-box capable of 

 exposing a surface sufficient to obtain a map of two degrees square, 

 and with instruments of large aperture we may hope to reach much 

 smaller stars than I have yet taken. There is also every probability 

 that the chemistry of photography will be very much improved and 

 more sensitive methods devised." 



In the light of recent work these words are almost prophetic. 

 The sensitive methods have been devised, and the result is that the 

 anticipations formed by Kutherford in 1864 are in 1886 not only 

 fulfilled but exceeded. It is in stellar photograj)hy that the 

 astronomer will be most benefited by the immense saving of labour 

 in making charts ; a single plate taken in one hour showing in their 

 proper relative place and in their relative photographic magnitudes, 

 all the stars down to the fifteenth magnitude over an area of about six 

 degrees— a result that it would be hardly possible to obtain by the 

 usual method of eye observation and measure. 



[Photographs of the stars round Altair, taken in 1883, of part of 

 Orion, taken in 1884 by the speaker, and some of the recent plates of 

 stars and double stars, by the Brothers Henry of Paris, were shown, 

 with a companion plate of similar parts of the sky as shown on 

 Argelander's maj)s, showing the enormous increase in the number 

 of stars shown by one hour's exj)0sure, in one case ten times as many 

 stars being on the photographic plate as on the same area of the map.] 



The i)lanets J upiter and Saturn were photographed by Dr. de la Eue 

 and others in the early days of photography. I have not been able 

 to obtain copies, or even a sight of any of these earlier ones, but I 

 have some of my own work that will enable you to see the improve- 

 ment that has been made since 1876. 



[Photographs of Saturn from 1877 and of Jupiter from 1878 were 

 shown, including some of the recent work of the Brothers Henry of 

 Paris, showing the great advance obtained by the enlargement of the 

 primary image.] 



With nebulae, although the work has been all done within the last 

 few years, the results have been very satisfactory. 



Dr. Draper, in 1880, obtained a very promising picture of the 

 Orion nebula, and I was able in 1883 (after trials commencing in 1879) 

 to get, with a three-foot reflector, some very fair photographs. ^ A com- 

 parison with the last drawing will show the chief points of difi'erence. 



Other nebulae have been photographed by myself and others, and 

 the power of the j)hotograph to portray these mysterious shapes has 

 been thoroughly demonstrated. 



