Qgi PHILOSOPHICAL TRANSACTIONS. [ANNO 1785. 



compose them must likewise perceive the same phenomena. For which reason 

 they may also be called milky-ways by way of distinction. My opinion of their 

 size is grounded on the following observations. There are many round nebulae, 

 of the first form, of about 5 or 6 minutes in diameter, the stars of which I can 

 see very distinctly ; and on comparing them with the visual ray calculated from 

 some of my long gages, I suppose, by the appearance of the small stars in those 

 gages, that the centres of these round nebulae may be 600 times the distance of 

 Sirius from us. In estimating the distance of such clusters I consulted rather the 

 comparatively apparent size of the stars than their mutual distance ; for the con- 

 densation in these clusters being probably much greater than in our own system, 

 if we were to overlook this circumstance and calculate by their apparent com- 

 pression, where, in about 6 minutes diameter, there are perhaps 10 or more stars 

 in the line of measures, we should find, that on the supposition of an equal scat- 

 tering of the stars throughout all nebulae, the distance of the centre of such a 

 cluster from us could not be less than 6"000 times the distance of Sirius. And 

 perhaps in putting it, by the apparent size of the stars, at tiOO only, I may have 

 considerably under-rated it; but my argument, if that should be the case, will be 

 so much the stronger. Now to proceed : 



Some of these round nebulae have others near them, perfectly similar in form, 

 colour, and the distribution of stars, but of only half the diameter : and the stars 

 in them seem to be doubly crouded, and only at about half the distance from each 

 other : they are indeed so small as not to be visible without the utmost attention. 

 I suppose these miniature nebulae to be at double the distance of the first. An 

 instance, equally remarkable and instructive, is a case where, in the neighbour- 

 hood of two such nebulae as have been mentioned, I met with a 3d, similar, re- 

 solvable, but much smaller and fainter nebula. The stars of it are no longer to 

 be perceived ; but a resemblance of colour with the former two, and its diminished 

 size and light, may well permit us to place it at full twice the distance of the 2d, 

 or about 4 or 5 times that of the 1 st. And yet the nebulosity is not of the milky 

 kind ; nor is it so much as difficultly resolvable, or colourless. Now in a few of 

 the extended nebulas, the light changes gradually, so as from the resolvable to 

 approach to the milky kind ; which appears an indication that the milky light of 

 nebulae is owing to their much greater distance. A nebula therefore, whose light 

 is perfectly milky, cannot well be supposed to be at less than 6 or 8000 times the 

 distance of Sirius; and though the numbers here assumed arc not to be taken 

 otherwise than as very coarse estimates, yet an extended nebula, which in an 

 oblique situation, where it is possibly fore- shortened by -J-, -§-, or -f of its length, 

 subtends a degree or more in diameter, cannot be otherwise than of a wonderful 

 magnitude, and may well outvie our milky way in grandeur. 



