THE GREAT STAR MAP ii 



Dr. Common and Dr. Richey have both succeeded in making 

 mirrors of 5 feet and a mirror of no less than 8^ feet diameter 

 is proposed ; but the largest lens in the world is the Yerkes 

 of 40 inches. Hence it could not fail to impress the conference 

 of 1887 that the more economical instrument would be a 

 reflector; moreover several such reflectors were already in 

 existence and could, so it was hoped, be utilised without 

 further expense. Thus at Oxford there was a reflecting 

 telescope, which Dr. De la Rue had presented to the University 

 Observatory, with which Prof. Pritchard hoped to take a 

 share in the great project : if it were decided to use a 

 different pattern of instrument his hopes would be dis- 

 appointed unless he could obtain the money necessary to 

 purchase one of the adopted pattern. 



As regards the two forms of refracting telescope, the 

 refractor and the doublet, that advocated by Prof. Pickering 

 was the more expensive and the less known. In the light of 

 our modern knowledge of its advantages (especially for the 

 purpose of covering a larger area of the sky at once) it is 

 very strange to find so little in support of it in the accounts 

 of the discussion. It seems to have been put aside almost at 

 once, in spite of the letter urging its adoption from Prof. 

 Pickering. The chief reason for this was undoubtedly lack 

 of information as to the accuracy with which plates taken by 

 such an instrument would give the places of the stars. Speci- 

 men photographs taken by the brothers Henry with the other 

 form of refractor had been measured and shown to be very 

 satisfactory, but there was no corresponding information about 

 the "doublet" as this third form of instrument is now usually 

 called. Hence the doublet was put aside from the start and 

 the choice was made between the reflector and the simple 

 refractor. 



The decision fell upon the latter. The choice has proved 

 to be a wise one and it is satisfactory to remember that 

 it was made without any acrimonious discussion. This was 

 largely due to Dr. Common himself, who might perhaps 

 have been expected to lay stress on the particular advantages 

 of his own special instrument. His experience however had 

 impressed him rather with its defects, especially with its 

 uncertainty. This uncertainty is not due to the instrument 

 itself so m,uch as to our fitful clirnate : the reflector is so 



