1886.] 071 Photography as an Aid to Astronomy. 371 



In the method of working there is one important difference between 

 that followed by the terrestrial and celestial photographer. \Yithout 

 exception, everything that the latter has to photograph is in continual 

 apparent motion, owing to the rotation of the earth, and in some cases 

 to the proper motion of the object itself. This necessitates the use 

 of an equatorial mounting to carry the photographic apparatus, with 

 clockwork to give it a regular motion in a direction contrary to that 

 of the earth. Even then the difficulties of keeping the telescope 

 moving for one or more hours without allowing deviation of the 

 image on the sensitive j)late of a yoVo ^^ ^^ mah. during this time, 

 taxes very severely the powers of the observer ; for, every such long 

 exposure must be watched not only to correct irregularities of the 

 clock, but other slight though important movements, due to change 

 of refraction and other causes, which, if not immediately corrected, 

 would spoil the picture. These mechanical difficulties, however, are 

 not insurmountable, as will be seen from some of the photographs I 

 shall show ; and as instruments improve and workers gain experience 

 they will become less. 



There are many technical details of extreme interest to the worker, 

 which it is hardly necessary to name to-night. 



The light from the different celestial bodies varies greatly in 

 intensity. Between that from our sun and that from the faintest 

 nebula or star that can be seen, there is such an immense difference, 

 that their relative amounts can hardly be expressed by figures. 



Dr. Huggins estimates the light of the faintest nebula that can 

 be seen with a moderately large instrument as equal to ^ooq-q ^^ *^® 

 light of a single standard candle viewed at a distance of a quarter of 

 a mile, that is, that such a candle a quarter of a mile off is 20,000 times 

 more brilliant than the nebula. 



The astronomer, who deals with both, has therefore need of all his 

 art to reduce the light in the one case to that suitable for his pur- 

 pose ; and to utilise every ray he can get in the other, regulating the 

 exposure for the one to a minute fraction of a second, and extending 

 for the other to hours. 



Between these two extremes of light-giving power are comprised 

 all other celestial objects. 



For the purpose of convenience I will take the photograiDhs, 

 which I propose to show you, in the following order : — (1) those 

 of the sun ; (2) the moon ; (3) the stars ; (4) planets ; (5) nebulae ; 

 and (6) comets ; giving in nearly every case an early photograph and 

 a recent one for comparison ; and, where I can, a specimen of the work 

 of eye and hand that may be directly compared with a photograph of 

 the same object. 



With the sun there are two distinct phenomena to observe: 

 (1) the physical aspect of his surface, with the remarkable spots 

 and markings that are frequently visible; and (2) the wonderful 

 prominences and corona that surrounds the sun and becomes visible 

 when he is eclipsed. 



