THE GREAT STAR MAP 7 



Southern. The dry plate had by this time made photography 

 easy and many members of the pubHc who had recently become 

 possessors of cameras essayed to photograph the comet ; they 

 found to their disappointment that the rotation of the earth 

 carrying them and their cameras with it was sufficient to 

 spoil their pictures. Thereupon Sir David Gill, then H.M. 

 Astronomer at the Cape, invited one of them to come to the 

 Observatory and to strap his camera to the equatorial telescope 

 (which was fitted with clockwork to counteract the earth's 

 motion) ; immediately some beautiful pictures of the comet 

 were obtained, and not only of the comet but of the surrounding 

 stars. The number of stars shown on the photographs was 

 indeed striking, and attracted widespread attention. The 

 late Dr. Common of Ealing, who had been constructing 

 telescopes for himself, without however any definite intention 

 of using them photographically, immediately turned them to 

 this new purpose and obtained some beautiful pictures of 

 nebulae. The brothers Henry in Paris saw the possibility of 

 substituting the new process for the immensely laborious 

 method by which they had been making their ecliptic charts; 

 but in their case the change could not be made so easily, as 

 their telescope had been made for visual use and could not 

 immediately be used photographically. The difficulty arises 

 from the existence of numerous colours in white light, the 

 colours with which we are familiar in the rainbow. When 

 looking through a telescope with the eye we use chiefly rays 

 nearly yellow in colour, whilst the photographic plate is 

 sensitive to blue and violet. Now a lens cannot be constructed 

 to focus all these rays at the same time and consequently for 

 photography a new lens must be made which will focus the 

 blue and violet light instead of the yellow. There are ways 

 of avoiding this difficulty which may be briefly mentioned. 

 In the first place if we use a mirror which brings the rays 

 to focus by reflection, instead of a lens which combines them 

 by refraction, no colour difficulty arises. (It was for this 

 reason that Dr. Common was able to use at once for photo- 

 graphy the reflecting telescope which he had originally built 

 for eye observation.) Secondly, modern improvements in the 

 construction of photographic plates have made them sensitive to 

 yellow light under certain conditions, so that visual telescopes 

 can be used to take photographs if a yellow screen cuts out 



