Andrew Ainslie Common. 315 



gave such a very bad image that it had to be condemned, and another 

 disc of glass made. Had it been just passably good, it is more than 

 probable that I should not have made another, but should have come 

 to the conclusion that the limit of useful size had been reached." He 

 is here speaking of the loss of two years' work, during which more 

 than two million strokes had been made with the grinding tools, and, 

 as he admits in another chapter, " the greater part of the period was 

 a time of worry and anxiety." But when he realised that it was 

 labour lost and he must begin afresh, he merely dwells with satis- 

 faction on the nett result that from the experience gained he was 

 able " to make the second disc into an almost perfect mirror in three 

 months." 



Unfortunately the telescope, though completed, has not been much 

 used. Some excellent photographs of nebulae were taken with it, and 

 only withheld from publication because Common felt he could improve 

 upon them. But while using it one night he narrowly escaped falling 

 from the high staging necessary to work a Newtonian reflector 

 such a fall would certainly have been fatal and he resolved that 

 he must not run a similar risk again. Consequently he decided that 

 the telescope must be re-arranged on the Cassegrain plan, so that the 

 observer might work in safety from the floor, and some years were 

 spent in experiments on the best method of doing this. Cassegrain's 

 actual design involves a central hole in the large mirror, and there 

 were known difficulties in the way. At first an attempt was made to 

 alter the plan to an " oblique Cassegrain," leaving the mirror intact ; 

 and promising results were obtained, which, however, never got 

 beyond a certain limit. Accordingly the known difficulties of making 

 a central hole were attacked and conquered, and a successful mirror 

 produced, so that the telescope is complete either as a Cassegrain or a 

 Newtonian. 



About this time, however, Common's attention was diverted to 

 problems concerning gun-sights and telescopes for the army, and this 

 was work which specially suited and attracted him. All observations 

 with the 5-foot came to a standstill, and he realised that he was 

 unlikely to resume them. An additional discouragement arose 

 from the deterioration in the atmospheric conditions, which proceeds 

 inexorably in the neighbourhood of London. Common felt that to 

 do the instrument justice it ought to be transported to a good climate, 

 and though plans for doing this were often vaguely in his mind, he 

 never crystallized them. His sudden death leaves standing in a 

 suburban garden, unused, what is probably the finest telescope in the 

 world. 



In the ordinary course of business Common had acquired consider- 

 able engineering knowledge, which helped him materially in the 



