66 TEE POPULAR SCIENCE MONTHLY. 



For such investigations new instruments are needed, of unexam- 

 pled powers and accuracy, some for angular measurements, some for 

 mere power of seeing. Photography comes continually more and 

 more to the front ; and the idea sometimes suggests itself that by- 

 and-hy the human eye will hardly be trusted any longer for observa- 

 tions of precision, but will be sujjerseded by an honest, unprejudiced, 

 and unimaginative plate and camera. The time is not yet, however, 

 most certainly. Indeed, it can never come at all, as relates to certain 

 observations ; since the human eye and mind together integrate, so to 

 speak, the impressions of many separate and selected moments into 

 one general view, while the camera can only give a brutal copy of an 

 unselected state of things, with all its atmospheric and other imper- 

 fections. 



New methods are also needed, I think (they are unquestionably 

 possible), for freeing time-observations from the errors of personal 

 equation ; and increased precision is demanded, and is being pro- 

 gressively attained, in the prevention, or elimination, of instrumental 

 errors, due to differences of temperature, to mechanical strains, and to 

 inaccuracies of construction. Astronomers are now coming to the in- 

 vestigation of quantities so minute that they would be completely 

 masked by errors of observation that formerly wcire usual and toler- 

 able. The science has reached a stage where, as was indicated at the 

 beginning of this address, it has to confront and deal with the possible 

 unsteadiness of the earth's rotation, and the instability of its axis. 

 The astronomer has now to reverse the old maxim of the courts : for 

 him, and most emphatically at present, de minimis curat lex. Resid- 

 uals and minute discrepancies are the seeds of future knowledge, and 

 the very foundations of new laws. 



And now, in closing this hurried and inadequate, but I fear rather 

 tedious, review of the chief problems that are at present occupying the 

 astronomer, what answer can we give to him who insists, Cui honof 

 and requires a reason for the enthusiasm that makes the votaries of 

 our science so ardent and tireless in its pursuit ? Evidently very few 

 of the questions which have been presented have much to do directly 

 with the material welfare of the human race. It may possibly turn 

 out, perhaps, that the investigation of the solar radiation, and the 

 behavior of sun-spots, may lead to some better understanding of ter- 

 restrial meteorology, and so aid agricultural operations and navigation. 

 I do not say it will be so — in fact, I hardly expect it — but I am not 

 sure it will not. Possibly, too, some few other astronomical investiga- 

 tions may facilitate the determination of latitudes and longitudes, and 

 so help exploration and commerce ; but, with a few exceptions, it must 

 be admitted that modern astronomical investigations have not the 

 slightest immediate commercial value. 



Now, I am not one of those who despise a scientific truth or princi- 

 ple because it admits of an available application to the affairs of what 



