494 LECTURE XLI. 



relative situations, and the directions of their paths may often render their 

 motions imperceptible to us. 



Respecting all these arrangements of stars into different systems, Dr. 

 Herschel has lately entered into a very extensive field of observation and 

 speculation, and has divided them into a number of classes, to each of which 

 he has assigned a distinct character. Some he supposes, like our sun, to be 

 insulated stars, beyond the reach of any sensible action of the gravitation of 

 others; and around these alone he conceives that planets and comets revolve. 

 Double stars, in general, he imagines to be much nearer to each other, so as 

 to be materially affected by their mutual gravitation, and only to preserve 

 their distance by means of the centrifugal force derived from a revolution 

 round their common centre of inertia; an opinion which, he thinks, is 

 strongly supported by his own observations of some changes in the positions 

 of double stars. Others again he supposes to be united in triple, quadruple, 

 and still more compound systems. A fourth class consists of nebulae like 

 the milky way, the clusters of stars being rounded, and appearing brightest 

 in the middle. Groups of stars Dr. Herschel distinguishes from these by a 

 want of apparent condensation about a centre of attraction; and clusters by 

 a still greater central compression. A seventh class includes such nebulae as 

 have not yet been resolved into stars, some of which Dr. Herschel supposes 

 to be so remote, that the light emitted by them must actually have been two 

 millions of years in travelling to our system. The nebulae of another de- 

 scription resemble stars surrounded by a bur, or a faint disc of light: a diffused 

 milky nebulosity, apparently produced by some cause distinct from the 

 mmediate light of any stars, is the next in order: and Dr. Herschel has 

 distinguished other more contracted nebulous appearances, in different states 

 of condensation, into the classes of nebulous stars, and planetary nebulae, 

 with and without bright central points. Many of these distinctions are 

 perhaps too refined to be verified by common observers ; but the discovery 

 of the existence of double and triple stars, revolving round a common centre, 

 will, if it be confirmed, add one more to the catalogue of Dr. Herschel's 

 important improvements. 



It is however fully ascertained, that some of the stars liave periodical 



