308 M. Arago on Ncbulw. 



properly studied till our own times. It has led to some mag- 

 nificent results respecting the constitution of the universe, 

 of which we shall soon make mention. At present we have 

 to do only with certain agglomerations of stars, which are local 

 and very circumscribed ; such, for example, as the group of the 

 Pleiades, the mass with which Q Argus is surrounded, or that 

 which has been remarked in the constellation Cancer, and 

 which bears the name of Prwsepe, &c. &c. 



To every short-sighted person, the Pleiades have the appear- 

 ance of a confused mass of light ; but when a glass is used, 

 which does not magnify, or the vision is rendered distinct by 

 simple spectacles, the principal stars of this group are seen 

 separately, and become detached, I may say, from one another. 

 The Pleiades, then, are a nebula only to certain observers, 

 and even that only when they do not use spectacles. In the 

 group of Cancer, the different stars being more condensed, the 

 naiural human vision cannot separate them ; the light of one 

 star becomes extended and scattered on the retina, mingles 

 with the light of the neighbouring star, on account of the im- 

 perfection of our organs, and the whole forms a confused mass. 

 Avail yourself, on the contrary, of a telescope, even of small 

 power, and the image of each star becomes greatly concen- 

 trated, is thus separated from the image of the contiguous star, 

 and the luminous mass loses the character of diffusion, which 

 can only be legitimately maintained in the class of true ne- 

 bulae. 



In order to attain this result, simple spectacles and a weak 

 glass have been found sufficient when we observe the Pleiades 

 and the group of Cancer. There are luminous spots which 

 we cannot resolve into groups of stars, but by the aid of the 

 best telescopes and strong magnifying powers. Those which 

 have resisted magnifiers of 50, 100, 150, and 200 times, give 

 way under magnifiers of 500, 1000, and upAvards. It was thus 

 that Herschel succeeded in transforming into agglomerations 

 of stars, the greater part of the nebulae which Messier, who 

 used less powerful glasses, believed to be irreducible, and which 

 he called nebulae without stars. 



Nature of Nebulce. — The considerable number of nebulae 

 which, when viewed with ordinary instruments, seem lumi- 



