PHOTOGRAPHY AND ASTROPHYSICS 439 



Possibly the second difficulty, that of preservation, may be an 

 antidote 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 mat- 

 ter 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 

 gathering, I think we might be well content with the result. 



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

 worthy of special remark because these particular funds are so often 

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

 the experience of any astronomer will supply him with plenty of 

 instances. The difficulties of publication have much in common 

 with those of storage; they will increase year by year, and even when 

 the money for printing has been found, the storage of publications 

 received from other observatories will itself become an embarrass- 

 ment. There is, however, one way in which some of the stress may 

 be relieved, namely, by efficient cataloguing. If we have before us 

 a list of all the photographs existing in the world, and know that we 

 can send for a copy of any one of them which may be required, it 

 is no longer necessary to have copies of all. This applies, of course, 

 to other publications as well; and though we may take some time to 

 grow out of the sentimental desire for a complete library, and though 

 the existence of a few such complete institutions may always be desir- 

 able, I venture to think that many observatories will ultimately be 

 driven to the plan of acquiring only what is certainly and imme- 

 diately useful, depending on temporary loans from central institutions 

 for other material. 



But there is a class of problems differing totally in character 

 from these practical questions of storage and preservation of plates. 

 A period of suddenly increased activity such as we have been passing 

 through in astronomy is not without important effects on astro- 

 nomers themselves. The human element in our scientific work is 

 sometimes overlooked, and generally accorded only a subordinate im- 

 portance; but, coming as I do from an old university devoted to the 

 Humanities, I may be perhaps forgiven for calling attention to a few 

 human considerations. In the first place, I have felt some anxiety 



