PHOTOGRAPHY AND ASTROPHYSICS 435 



of a photograph was doubted. Even now it can scarcely be said 

 that we know definitely the stage of refinement at which we must 

 begin to expect irregular displacements of the images from dis- 

 tortion of the photographic film; but we have learned that they do 

 not occur in a gross degree, and that other apparatus must be im- 

 proved before we need turn our attention seriously to errors arising 

 from such a cause. Consider, for instance, what photography has 

 told us about our optical apparatus, which we regard as having 

 reached a high stage of perfection. We are accustomed to think 

 of properly made optical apparatus as being sufficiently similar in 

 all its parts; it is tacitly assumed in the principle of the heliometer, 

 for example, that one half of the object-glass is sufficiently similar 

 to the other. But a stock adjustment recently adopted in photo- 

 graphing a spectrum for accurate measurement exhibits clearly the 

 errors of this assumption. Photographs are taken of the spectrum 

 through the two halves of the objective; and if they were properly 

 similar the lines in the two halves of the spectrum should fit exactly. 

 A mere glance is usually sufficient to show discordances. It is true 

 that one of the photographs is taken through the thick half of the 

 prism and the other through the thin, so that errors of the prism are 

 included; but these, again, are optical errors. They are, however, not 

 the only sources of error which at present mask photographic imper- 

 fections. Glass plates are not flat, and this want of flatness intro- 

 duces sensible errors. Even with the great improvements in our 

 driving-clocks which were called for immediately photographs were 

 to be taken, with electrical control and careful watching on the 

 part of the observer, there is apt to creep in a "driving-error " 

 which gives bright stars a spurious displacement relatively to faint. 

 We must get flatter plates, better driving-clocks, and watch more 

 carefully before we can certainly accuse our photographs of a failure 

 in accuracy. Nevertheless, there are indications that we may be near 

 the limit of accuracy even now. Examination of the re"seau lines on 

 various plates appears to show small displacements for which no cause 

 has yet been assigned; and the end of our tether may not be far 

 away. But as yet we have not been pulled up short, and there is hope 

 that the warning may be, as on one or two previous occasions, a false 

 alarm. 



Such being the accuracy of the photographic method, it is surprising 

 that it should not as yet have been more fully adopted in that field 

 of work where accuracy is of the greatest importance, namely, in 

 what is called fundamental .work, with the transit-circle or other 

 meridian instruments. The adoption of new methods is always a 

 slow process, and there are at least two classes of difficulties which 

 hinder it. The first class has its origin in the instinctive conservatism 

 of human nature, wherein men of science differ little from their 



