2-44- ■^'^' Herschel on the 



T 

 half the given field of view, it will be - B, the bafe of the 



cone. And — - — rrj, will be an expreffion iov vp, in terms 



of V5^ which is the mutual ditlance of tlie fcattered ilaps. 



Then having — rr ;2' -f | «*+ I ;z, we may find ?} ; whence 



zdn - d, the vifual ray, will be obtained. 



The refult of this arrangement gives a fiiorter ray than that 

 of the former ; but fince the dlfixrence is not fb confiderable as 

 very materially to afFc£l the conclaiions, I fliall, on account of 

 the greater convenience, make uie of the hrft» 



We inh^ihit the planet of a Jlar hAonging to a Compvund Nebula 



of the third form. 



I fhall now proceed to fliew that the ftupendous fidereal fyiLem 

 we inhabit, this extenfive flratum and its fecondary branch, 

 confifting of many millions of ftars, is, in all probability, a 

 detached Nebula. In order to go upon grounds that feem to me 

 to be capable of great certainty, they being no lefs than an 

 actual furvey of the boundaries of our fidereal fy ftem, which 

 J have plainly perceived, as far as I have yet gone round it, 

 every where terminated, and in mofl places very narrowly too, 

 it will be proper to (hew the length of my founding line, if I 

 may fo call it, that it may appear whether it was fufficiently 

 long for the purpofe. 



In the moft crowded part of the milky way I have had fields 

 of view that contained no lefs than 588 ftars (^), and thefe were 

 continued for many minutes, fo that in one quarter of an hour's 

 time there palled no lefs than 1 16000 ftars thiough the field of 



{'i) Sec the table of Gages, p. 235. 



view 



