penetrating into Space by Telescopes. 63 



ought to be a visible -general difference between stars of one 

 order and that of the next following, I think, from the faintness 

 of the stars of the 7th magnitude, we are authorized to conclude, 

 that no star, eight, nine, or at most ten times as far from us as 

 Sirius, can possibly be perceived by the natural eye. 



The boundaries of vision, however, are not confined to single 

 stars. Where the light of these falls short, the united lustre of 

 sidereal systems will still be perceived. In clear nights, for 

 instance, we may see a whitish patch in the sword-handle of 

 Perseus,* which contains small stars of various sizes, as may be 

 ascertained by a telescope of a moderate power of penetrating 

 into space. We easily see the united lustre of them, though the 

 light of no one of the single stars could have affected the unas- 

 sisted eye. 



Considerably beyond the distance of the former must be the 

 cluster discovered by Mr. Messier, in 1764; north following 

 H Geminorum. It contains stars much smaller than those of 

 the former cluster ; and a telescope should have a considerable 

 penetrating power, to ascertain their brightness properly, such 

 as my common 10-feet reflector. The night should be clear, 

 in order to see it well with the naked eye, and it will then 

 appear in the shape of a small nebula. 



Still farther from us must be the nebula between y and £ 

 Herculis, discovered by Dr. Halley, in 1714. The stars of it 

 are so small that it has been called a Nebula ; -f and has been 

 regarded as such, till my instruments of high penetrating 



* See the catalogue of a second thousand of new nebulae and clusters of stars, VI. 

 33, 34. Phil. Trans. Vol. LXXIX. page 251. 



f In the Connoissance des Temps for 1783, No. 13, it is described as a nebula 

 without stars. 



