13H7 



Thus the gradients become 0,4^^ 0,30 and 0,23. These nnnibeis 

 again sliow no ti-aee of' a spacial condensation in distant galactic 

 clouds. 



The conti-adiction that appears in all tiiese results, and that has repeat- 

 edly disappointed the hope of jienetraling fnrthei' than Hkrschki-, can be 

 summarised tiius: /// llw hrufJit ijdlactic cIoiuU the Fratil-lhi- Adams 

 plates sJioir hnrdli/ aiitj (jreater amoiint of stars than did the (/aia/es 

 of Hkksc'HEI,, although, as tar as (he avei'age numbers are concerned, 

 tliev go far deeper. On the regi(>n of plale 136 the countings of 

 Chapm.-vn and Melottk give 9340 stars per sipiai-e degree, and Hkkschfj. 

 7500, whereas the average of the entire galactic zone with the one 

 surpasses 2800, and with (he othei* only amounts to l()2o. 



it is not immediately clear what «nay be the cause heieof. The 

 most plausible explanation is, that the countings of the faintest 

 stars on the Franklin- Adams plates in the densest regions are far 

 more incomplete than in other regions. Another explanation would 

 be, that in (he bright, dense galactic clouds (he colour-index is 

 higher, so (hat there (he average of the stars would be redder than 

 in the average of the galactic zone. In this case with coun(ings on 

 photographic plates, no matter how complete, we advance less than 

 with visual countings by means of a telescope with a wide opening. 

 So far therefore we cannot |)enetrate further in(o the depths of (he 

 galactic clouds than Herschkl did; our material reaches hardly any 

 further than that collected by Wiu.iam ItIkrschkt. more than a century 

 ago. That nothing has been done during the whole of (he 19^'' 

 century to complete and correct his work, is doubtlessly due (o the 

 fact that the photographic method with regard (o (he coundng of 

 s(ars promised so much more, bu( has failed as ye( to fulfill its 

 promise. The numerous systematic ditïerences which the photogra|)hic 

 method involves, — the decrease of star-density towards the borders, 

 the greater influence of atmospheric absorption, the vaiiation in 

 limiting magnitude • — all this renders it extremely difficult, 

 to deduce a homogeneous material from a |)hotographic survey of 

 the sky. If we consider, moreover, that the faintest stars, the main 

 object of investigation, as an average have a higher colour-index, it 

 becomes yet more evident how desirable visual countings with 

 instruments of high power are for (he s(udy of the galalic con- 

 densations. 



