10 SCIENCE PROGRESS 



time. On such occasions the chairman rings a bell and 

 declares the sitting intermitted for a few minutes. What 

 has been public discussion can now be developed as private 

 conversation. Gentlemen of opposite views who have been 

 addressing one another excitedly across the width of the room 

 may now rush together and arrive at a better understanding 

 at close quarters. The effect of such an opportunity soon 

 becomes evident when after a few minutes' interval the chair- 

 man again rings his bell — a calm has succeeded to the 

 storm and not infrequently it is possible to crystallise out a 

 resolution. 



Let us glance at one or two of the matters which had to 

 be decided in 1887. The first and most important was the 

 choice of an instrument or instruments — for it was a pre- 

 liminary question whether the same pattern should be used 

 by all those co-operating in the work. This preliminary 

 question, however, was soon settled in the affirmative. All 

 were to use similar instruments ; and now what were they to 

 be ? Should they be reflecting telescopes as used by Dr. 

 Common, refracting telescopes as made by the brothers Henry, 

 or refracting telescopes of a different pattern and more closely 

 similar to camera lenses as advocated by Prof. Pickering of 

 Harvard ? 



The advantages of the reflector were that it was cheap 

 and that it existed. It is cheap because there is only 

 one surface to be polished. Reflectors used to be made of 

 speculum metal polished to a concave form ; such were, for 

 example, the great telescopes of Sir William Herschel and of 

 Lord Rosse : nowadays instead of metal we use glass silvered 

 on the face (not on the back as in a domestic looking-glass) : 

 but in either case there is only one surface to be prepared 

 optically. Now with lenses there are two, four, or even more 

 surfaces, all of which must be optically true. Moreover the 

 glass must be entirely free from blemishes ; if there is a 

 fault in the substance of the glass which forms a mirror it is 

 behind the reflecting surface and may not spoil the image but 

 a fault in the interior of a lens cannot fail to produce its 

 effect. Hence a lens is always much more costly than a 

 mirror of the same size and the greatest telescopes in the 

 world have always been reflecting telescopes. Lord Rosse's 

 6-foot mirror has not yet been surpassed in size, although 



