PHOTOGRAPHY IN ASTRONOMICAL RESEARCH. 179 



unscientific to take more photoi^raphs than can he deaU with imnie- 

 diately. This seemed to he a phiiisihle ai"<>nment and to sliow a way 

 ont of the diflTiculty. for if a ])hoto<:Tai)h had once been ade(|uately 

 examined it need not he stored so carel'idly. and there woidd not in 

 any case be many to store. P>ut Professor Pickerinii' has demon- 

 strated many times ovm- tlial tiie vi»Mv is untenable. P>y taking- photo- 

 graphs abnost recklessly and without any hope of dealino; with even 

 a fraction of them, he has created the ])ossibility of tracing- the his- 

 tory of celestial events backward. AVhen new objects are discovered 

 he can go to his shelves and t(dl us how long they were visible previous 

 to disco\'ery ; and this information is so valuable that we must cer- 

 tainly arrange our future i)lans with reference to it. It is quite cer- 

 tain that we nn]st l)e pi'ei)ared to deal with enormous accunndations 

 of phites, to store theui iu ])ropei' order, and to catalogue them; and 

 if it has already been found (htlicult to do this for the collection of a 

 single observatory dui-ing twenty years, what can we look for in the 

 centuries to come 'i 



Possibly the second diHiculty, (hat of ])reservation, nuiy be an anti- 

 dote to the first. It is by no means certain that our photographs 

 will last long, and if not there will be a natural limit to the time 

 during which they need be kept. Sir William Crookes has, however, 

 reminded us that by toning them, Ijy sul)stituting sturdy gold for the 

 l^erishable silver, we may prolong their life indefinitely, though this 

 will, of course, sensibly increase the cost of each plate. As yet I have 

 not heard of any toning i)rocess being systiMuatically adopted. Our 

 course is, however, comparatively clear in this direction. It would 

 seem imperative that a selection of the earliest photographs, at any 

 rate, should l)e carefully toned, so that they may be available for 

 comparison in years as far distant as j>ossible. Although this is a 

 matter of detail, it seems to me to compare in importance with almost 

 any practical question which may claim the attention of astronomers, 

 and if some decision of the kind Avere the only outcome of this gath- 

 ering I think Ave might be Avell content Avith the result. 



The question of puljlication is chiefly one of funds, and is only 

 Avorthy of special remark because these particular funds are so often 

 forgotten in ])lanning ent(u-i)rises. I need not labor the point, for 

 the experience of any asti'ouomer will su])})ly hiui with plenty of 

 instances. The difficulties of publication have unich in connnon Avith 

 those of storage. They Avill increas(> year l)y year, and even Avhen the 

 money for printing has becu found the storage of publications iv- 

 ceiA^ed from other observatories will itself become an eiubarrassment. 

 There is, hoAvever, one Avay in which some of the stress nuiy be 

 relieA'ed, namely, by efficient cataloguing. If we have ix^fore us a list 

 of all the i)hot()graplis existing in the vAorld, and know that avc can 

 send for a copy of any one of them Avhich may be required, it is no 



