9 1 



FIG. 12. 



This figure should be examined with a magnifying glass. It is a 

 direct reproduction of a photograph of a detached nebula and sur- 

 rounding stars in Cygnus by Dr. Max Wolf of Heidelberg (reproduced 

 by permission from the Monthly Notices of the Royal Astronomical 

 Society, vol.. Ixiv, Plate 18, p. 839, q.v.). The exposure was four hours on 

 July ioth, 1904, with a camera the lenses of which have a diameter of 

 sixteen inches. The picture is enlarged so that the apparent diameter of 

 the Sun or Moon would be about i inch on the same scale (one minute, 

 or sixtieth of a degree, equals one millimetre). 



The "apparent diameter" of the sun or moon is about one in 115: 

 that is to say that a covering disc of any size you like can be made 

 exactly to coincide with and " cover " the disc of the sun or moon provided 

 that you place it at a distance from the eye equal to 115 times its own 

 diameter thus a disc of an inch in diameter (say a halfpenny) will just 

 "cover" the sun or moon if placed at a distance from the eye of a little 

 less than ten feet, a threepenny piece will cover it at about six feet, and a 

 disc of somewhat less than half that size when held at arm's length. 



The nebula (on the horizontal A A) is seen surrounded by a dark 

 space at the end of a long dark lane or "rift" which reminds us of the 

 track left by a snowball rolled along in the snow. Has the nebula in some 

 mysterious way swept up the stars in its journey through space ? We 

 cannot at present either affirm or deny such interpretations. 



One or two of the brightest of the surrounding stars might just be seen 

 by an acute eye unaided by a telescope but no more. The best existing 

 telescopes would show only the large nebular body on the line A A) and 

 the larger white spots ; the finest dust-like particles are stars of which the 

 existence is only demonstrated by prolonged photographic exposures such 

 as this, with a lens which focuses its image on to the dry plate. The old 

 " wet-plate " would not remain wet sufficiently long to " take " the picture. 



It should be borne in mind in looking at this picture that each of the 

 minutest white spots is probably of at least the same size as our own sun : 

 further, that each is probably surrounded by a planetary system similar 

 to our own. 



