PHOTOGRAPHY IN ASTRONOMICAL RESEARCH. 179 



unscientific (o take nioic pliotoiii-aplis than can bo dealt with imme- 

 diately. This seemed ((» l)c a plausible ari»"ument and to show a way 

 out of the difliculty. for if a ph()toiiTa[)h had once been adecjuately 

 examined it need not be stored so car(d"ully, and thei-e would not in 

 any case be many to store. But T^rol'essor Pick'erin<i' has demon- 

 strated many times o\''r that the \i»'w is untenable. Ry taking photo- 

 <>rai)hs almost recklessly and without any hope of dealinc; with even 

 a fraction of them, he has created the possibility of tracing the his- 

 tory of celestial events backward, ^^dlen new objects are discovered 

 he can go to his shelves and tell us how long they were visible previous 

 to discovery; and this information is so valuable that Ave must cer- 

 tainly arrange our future plans with reference to it. It is quite cer- 

 tain that we must be pi'epared to deal with enormous accumulations 

 of plates, to store them in pro[)er order, and to catalogue them; and 

 if it has already l)een found diflicult to do this for the collection of a 

 single observatory during twenty years, what can we look for in the 

 centuries to come? 



Possibly the second difficulty, that of preservation, may 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, by substituting sturdy gold for the 

 perishable 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 process being systematically 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 be carefully toned, so that they may be available for 

 comparison in years as far distant as possible. 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 were the only outcome of this gath- 

 ering I think we might be well content with the result. 



The question of i)ul)lication is chiefly one of funds, and is only 

 Avorthy of special remark because these particular finids are so often 

 forgotten in planning enterprises. I neetl not labor the point, for 

 the experience of any astronomer Avill supply him with plenty of 

 instances. The difficulties of publication lun-e much in conmion Avith 

 those of storage. They Avill increase year by year, and even Avlien the 

 m(mey for printing has been found the storage of publications re- 

 ceived from other observatories Aviil itself become an embarrassment. 

 There is. hoAvever, one Avay in Avhich some of the stress may be 

 relieA'ed, namely, by efficient cataloguing. If Ave haA^e before us a list 

 of all the photographs existing in the world, and knoAv that we can 

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



