12 SCIENCE PROGRESS 



seriously influenced at times by air currents and changes of 

 temperature as to be an instrument of moods and Dr. Common 

 has accordingly compared it, somewhat ungallantly, to the 

 female sex. He himself took the initiative in recognising that 

 the Conference should adopt for a work of such magnitude 

 the more trustworthy refractor as made by the brothers Henry ; 

 this straightforward course had its due effect on the formu- 

 lation of a decision. There are now therefore a score of 

 such instruments scattered about the world, varying a little 

 in non-essentials but all closely resembling one another in 

 the size of the lens (which is 13I inches in diameter) and 

 in the focal length of the telescope (which is about ii| feet). 

 The focal length is actually defined to be that which represents 

 one minute of arc by a millimetre on the photographic plate ; 

 and this relation is so useful that in cases where a larger 

 telescope has been built, the relationship has been recognised 

 by making the scale exactly twice the size. Dr. Common 

 adopted the same focal length (of about 1 1 feet 6 inches) for 

 his excellent mirrors of 30 inches aperture ; with these recently 

 the beautiful photographs of comets have been taken and their 

 power of discovering faint satellites has also been shown. 



Another very important decision taken by the Conference 

 of 1887 had a rather curious history. It arose from the 

 ignorance, at that time, of the behaviour of a photographic 

 film and the fear lest it should shrink in drying or otherwise 

 become distorted. Experience of photography generally — as for 

 instance the taking of portraits or landscapes — was sufficient 

 to show that such distortion was at any rate not large ; but 

 in astronomy we are concerned with very minute quantities 

 and it was not known whether minute disturbances might not 

 affect the relative positions of the images on the plate. Accord- 

 ingly it was proposed to imprint upon each plate a series of 

 accurately ruled cross-lines called a resean. They were to be 

 photographed on the plate before development by exposing it 

 to an artificial light behind a silver matrix (a flat plate coated 

 with silver ruled with such lines) ; on development the lines 

 appear together with the star images and if the film has shrunk 

 during any of the processes of development, fixing, washing, 

 etc., these lines will have shrunk sympathetically and will be 

 no longer straight or at exactly equal distances as they were 

 in the matrix, We have now learned that such shrinkage is 



