2G4 



Dr. G. W. Royston-Pigott. 



limit of visibility measured by visual angle is readied, equally by the 

 eye-lens of a microscope or telescope, or by a simple magnifying glass, 

 when the object subtends too small an angle to be sensible to the 

 retina.* Both instruments require identical eye-pieces. (Two seconds 

 of arc is the limit in general of human vision.) 



Comparing, then, the minimum visible in both instrnments, and 

 beginning with the telescope, it may be urged that the spurious disks 

 of the minutest stars visible, such as the 17th or 20th magnitude, 

 must subtend an exceedingly small visual angle. 



It is then fair to inquire under what visual angle such disks can be 

 glimpsed in an eye-piece, used indifferently in either microscopef or 

 telescope. Great loss of light in defining these minute points is 

 prevented by very accurate curvature in a reflecting mirror. £ Accord- 

 ingly the "[Iranian satellites have been seen in an 18- inch silver glass 

 reflector. The extreme minuteness of these bodies is illustrated as 

 follows : — 



A double star between (3 l and /3 2 Oapricorni may be discovered whose 

 components are of the 16th and 17th magnitude. Yet Sir J. Herschel 

 declares, that in comparison with the Uranian satellites, these two 

 minute stars are splendid objects. The visual angle at which these 

 satellites are seen in Calver's 18-inch mirror must be exceedingly 

 small indeed§ (see mercurial double stars, fig. 9). 



Sir John Herschel records an extremely fine observation of the 

 double star 2 2 , composed of two minute equal stars nearly as small 

 as the 9th magnitude ; seen at the Cape with a 20-feet reflector armed 

 with a 24-inch mirror : — 



" Charmingly divided with 320 ; the disks, like two grains of 



* The spurious disks of stars of excessive minuteness are very much smaller 

 than those of the larger kind. But the usual mathematical expression for this size 

 is independent of star magnitude, and is in this respect incomplete. Such disks 

 are therefore matters of observation, and their size in some degree conjectural. 



f The mercurial illuminated star disks are here alluded to. 



X The same economy of light is secured in microscopic object-glasses of great per- 

 fection, which disperse the otherwise inevitable white fog of spherical aberration. 

 The visibility of these Uranian satellites in Calver's 18-inch silver glass mirror is 

 doubtless due to a similar cause. 



§ Compare this with the satellites of Saturn, 280,000,000 distant : — Dione : 

 diameter, 500 miles ; visual angle, 0""85 ; magnitude, 12. Tethys is reckoned mag- 

 nitude 13 ; E?iceladas, 15 ; Mimas and Hyperion, 17. If the law could be applied, 

 that the light being the same, visibility varies as the square of the diameter, and the 

 next magnitude is half the magnitude preceding, the visual angles subtended by 

 the Saturnian satellites might be guessed at with a power of 500 : — 



Rhea. Dione. Tethys. HJnceladus. Mimas and Hyperion. 



Magnitude 10th 12th 13th 15th 17th 



Visual angle .. .. 160" 67" 47" 23" 16" 



And this very nearly agrees with the Emits assigned. 



