176 LECTURES. 



at twice tlie distance is four times as feeble, &c., it will be seen tliat a 

 telescope which would increase the intensity of light to four times that of 

 the light seen by the bare eye might enable us to see twice as far, &c. 



By making use of a telescope of a greater and greater space-pene- 

 trating power Sir William Herschel, in investigating portions of the 

 milky way, continued to see new stars up to the twenty-eighth 

 order of distance. The borders of the milky way are supposed to 

 be at the nine hundredth order of distance. If this be so the 

 time of the arrival of light from the borders of the milky way must 

 not be measured by a single year, but by centuries ; in fact, so far as 

 we may rely on the conclusions of Dr. Madler, the distance of the 

 centre of this our group from us, as thus estimated, is 537 years. He 

 concludes, moreover, that the stars in the milky way and our sun 

 with them revolve at the rate of once in about eighteen million years. 

 "Whether we regard this as accurately ascertained or not, very certain 

 it is that the sun and ell planets are moving in the regions of space. 



The researches of Herschel, Argelander, Struve, and others, have 

 all contributed to point out very accurately a single spot in the heav- 

 ens, towaids which we are incessantly travelling by a motion very 

 slow when we consider the magnitude of the orbit, the distance trav- 

 elled being about four-fifths of the diameter of the earth's orbit every 

 year. When we scrutinize the outskirts of the milky way and at- 

 tempt to see beyond it, we find what seems to be an entirely detached 

 combination of stars. If what we see in them be stars only about the 

 size of those in the milky way we might readily conclude that they 

 were at no greater distance ; but it may be that what are apparently 

 single stars are themselves combinations. These groups are called 

 clusters. This is the representation of a coarse cluster. We find others 

 much more closely arranged, as in the figure, where they are repre- 

 sented by a white, powdery substance. The stars near the centre are 

 not to be counted by hundreds. When clusters become so remote that 

 you cannot make out the individual stars you may still discern clus- 

 ters of a granular shape and appearance in their structure ; or that 

 they are made up of a "star dust," an expression sublime from 

 its very simplicity. In this quasi crystalline mass the molecules are 

 double stars, the ultimate particles are suns, and the atoms, if any, 

 are planets. If the cluster be a globular one it may also be true that 

 all the stars, the outer ones only excepted, are revolving around the 

 centre in the self-same time. Beyond these still are the nebulaB, some 

 of which the most powerful telescopes have failed to resolve; that is, 

 have i'ailed to show that they are made up of stars. In other cases 

 they are found to be made up of stars, and resolvable. We cannot 

 positively assert that there is no cloudy-looking substance existing 

 in the heavens which is not made up in this way ; some appear- 

 ances, surrounding stars, cannot as yet be resolved. Other whole 

 nebula3 cannot yet be resolved by telescopes of large space-penetrating 

 power. Some idea of the distance of a nebula not resolvable may be 

 obtained by ascertaining the space-penetrating power which will cause 

 that nebula to present the appearance put on by another before power 

 suflScient was applied to resolve it; and thus, comparing the powers 

 employed in the two cases, we arrive at a distance so great as that a 

 comparison by means of the velocity of light itself becomes almost 



