PFlOTOdKATHY IN ASTRONOMICAL RESEAROH. 175 



Avhether a (leliiiltc coiu'liisioii would ever have been i-caclu'd at all. 

 By the sheer facility ^of the new method ol" work we have advanced 

 by leaps and bounds where wi' could only ci'awl before. 



Thirdly, tliere has been a u'reat e-aiu in accuracy from llie intro- 

 duction of ])hot()ii'i'a])hy ; and it is this (|iiality which is above all of 

 A'alue in the science of astronomy." T\n' wonch'rl'ul exactness of the 

 photogTa]:)]iic recoi-d may |)erhaps best be chnracterized by saying 

 that it has re\('ale(l the deficiencies of all our othei- astronomical 

 apparatus — object glasses and ])risms, clocks, even the observer him- 

 self. 



It has almost been forgotten that in the early days the accnracy 

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

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

 begin to expect irregular displacements of the images from distor- 

 tion of the j)hotogra])hic film; but we have learned that they do 

 not occur in a gross degree, and that other apjjaratus nuist be im- 

 proved before we need turn our attention seriously to errors aris- 

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

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

 reached a high state of perfection. A\'e are accustomed to think of 

 properly made optical ajjparatus as being sufliciently 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 sufliciently similar to 

 the other. But a stock adjustment recently adopted in photograph- 

 ing a spectrmn for accurate measurement exhibits clearly the errors 

 of this assumption. Photographs are taken of the specli-um through 

 the two halves of the objective; and if they wei'e properly similar 

 the lines in the two halves of the s|)ectruni should Ht exactly. A 

 mere glance is nsually 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 in- 

 cluded; but these, again, are optical errors. 'I'hey are, however, 

 not the only sources of error which at ])resent mask photographic 

 imperfections. (Ilass plates are not flat, and this want of flatnx'ss 

 introduces sensible eri'ors. Even with the great im|)i-o\-ements in 

 our driving clocks which were called for iunnedialely photogra])hs 

 Avere to be taken — Avith electrical control and carid'ul watching on 

 the part of the observer — there is ap)t to creej) in a " dri\ing error*" 

 which giA'es bright stars a s])U)'ious displacement relati\'ely too faint. 

 We must get flatter plates, better driA'ing clocks, and watch more 



« Two things may be measured on a pbotograpliic i>I;ite the position of an 

 object, or the density of the ini.-ige; tlio formei' Iteiiig an iiidic-ilioii of its jxisi- 

 tion in the heavens, and llie latter of its bri,t,'liliiess. With the Litter topic I 

 do not propose to deal, for the reason that it is in the hands of a much abler 

 and more experienced exponent ; but the former ahnie will provide enough food 

 for reflection. 



