316 Obituary Notices of Fellows deceased. 



construction of his instruments. Many of his scientific papers contain 

 suggestions of great value for the improvement of instruments. 

 Thus he pointed out that when taking a photograph with a large 

 telescope any small defects of clock-driving should be corrected, not 

 by moving the whole telescope, which will introduce strains and set 

 up oscillations, but by moving the plate alone. 



He suggested that the heavy parts of big instruments should be 

 floated, and carried out the idea beautifully in the case of his 5-foot. 

 There is a paper of his (" Mon. Not. K.A.S.," vol. 44, p. 288) on " Im- 

 provements in the construction of large transit circles," which has 

 attracted little attention, but which contains several suggestions of 

 obvious value and importance. 



He was thoroughly appreciative of the work of others, especially 

 when it lay in fields familiar to him. Lord liosse's work on the 

 nebulae was constantly referred to by Common in terms of warm 

 admiration ; when he presented his successful photograph of the Orion 

 nebula to the Eoyal Astronomical Society, as above mentioned, he 

 characteristically took the opportunity of referring to Lord Rosse 

 with enthusiasm. Foucault, who had developed the plan for testing 

 mirrors, was another of Common's heroes. And his was no mere idle 

 appreciation he carried it to a practical issue by carefully studying 

 what others had done before he commenced his own operations. " A 

 plan of work was sketched out," he writes in the introduction to his 

 account of the making of the 5-foot, " in which an endeavour was 

 made to adopt, as far as circumstances permitted, all the essential 

 things that had been pointed out by previous workers." At the same 

 time, he was too courageous to be fettered by the opinion or practice 

 of his predecessors, however impressively unanimous. " Every well- 

 known worker hitherto, without exception, has begun by making small 

 mirrors, and, as experience and skill were gained, has increased the 

 size of the discs." The weight of testimony was treated by Common 

 with respect, but ultimately disregarded when there seemed to be good 

 reason for a more enterprising course. 



The skill acquired in the difficult art of grinding mirrors was used 

 for the general good with unstinting generosity. Common made a 

 large number of mirrors at different times, which were presented to 

 colleagues, or put at their disposal for the minimum cost. He made 

 not only concave but plane reflectors, such as those required for ccelo- 

 stats used on British eclipse expeditions. Indeed, his help with these 

 instruments at a critical time was undoubtedly the means of intro- 

 ducing them to the attention of astronomers, who have since learnt 

 more of their great value. Two concave mirrors of 20 inches aperture 

 and 45 inches focus, made for eclipse work and used in 1889, are also 

 well worthy of special mention. 



