IMIOTnORAI'HV IN ASTKONOM K'A I- KKSEAKCH. 173 



oven (lie spcct i-iiiii (if this I'aiiit object has lu'cii photojrraplicd. When 

 we consider (liat in. (lie spectnmi each point of li,i>ht in the object is 

 enonnonsly diluted by bein<j^ spread out into a Hni', the dillicuhy of 

 (his undertakin<i' seemed ahnost pi-ohibitive; but it was not sullicient 

 to prevent Mr. Perrine, of the Lick Observatory, from makinii; the 

 attempt, and lie was (U'serxcdly rewarcU'd by success. 1 may b(! 

 wrong in regarding this success as the high-water mark in this direc- 

 tion at the present time, and it will i)robably be surpassed by some 

 new achievement \ery shortly; but it will serve to illustrate the 

 power of photography in dealing with faint objects. 



But may we here pause for one moment to marvel at the sensi- 

 tiveness of the human eye, which is such that it is, after all, not 

 left very far behind in the race? The eye, sensitive as it is mei-ely 

 to transient impressions, is no match ultimately for the plate, which 

 can act by accumulation. But with similar instruments the plate 

 nuist be exposed for minutes or even hours to seize the impression 

 of a faint object which the eye can detect at a glance. Th<n-e seems 

 to be no reason in the nature of things why the eye should not have 

 been surpassed in a few seconds; and in the future the sensitiveness 

 of plates may be increased so that this will actually be the case, even 

 as in the past there was a time when the sensitiveness was so small 

 that the longest exposure could not compete with the eye. But this 

 time is not yet come, and at the jiresent moment the eye is still in 

 some departments superior to its rival, owing to this very fact, 

 that though it can only see by glances, it can use these glances to good 

 efiect. In the study of the planets the more clumsy method of the 

 photographic plate (which, by re(iuiring time for the formaticm of 

 the image, confuses good moments with bad) renders it almost use- 

 less as compared with the eye; and again, Ave have not as yet used 

 photography for daylight <)bser\ ations of stars. 



But there is another directi(m in which th(> photogi'aphic plate is 

 immensely superior to the eye in power; it can record so nuich more 

 at once." In the able hands of Professor Barnard, Dr. Max Wolf, 



oThis property lias been l)eautifully ilUistrntivl by n Icctun' cxiicriint'nt of 

 Professor Barnard. He tbrows on Ibc screen :i pictnre of a \:\v^v nebula 

 whuli the i)liotot,M-a|>iii<- l»late lias no (lllliculty in poi-trayin.^ all at once; but 

 the j)icture is, in the (irst instance, covered up by a screen, exce|it for a small 

 Mlierture only, a!id this ai>erture, he tells his audience, represents ail that can 

 be seen by the eye at one time, iisin.n the .uiant telescope (»f the Verlces Observa- 

 tory r.y moviuix the screen about, diflei-ent i)ortions of the picture may be 

 viewed successively, as also liy niovini,' the telescope about in lookinji at the sky 

 itself, r.ut wliat a revelation follows wlieti the screen is i-emoved .and the full 

 irlory of tlic nebula is exhibited at a single .i,'lance: We can well understand 

 that the true character of these objects was hoi)elessly misinterpreted by the 

 eye usins the imperfect method of piecemeal observation, whiih alone was 

 formerly possible. 



