242 REPORTS FROM THE SECTIONS. 
comparison. He found a marked and ain regular increasé in 
the apparent diameter of the disc as the aperture of the telescope 
is diminished, thus agreeing with the results derived a fortiori from 
theory. For Sonat the es of a nes pea were— 
Aperture 7 in'33—6 in‘0—4 in95 4 in‘00-—3 in‘00—2 in ‘00 
Size of dise 0” 731" *807—0" "901-1" *284—1”- $29 2”°435 
(“Monthly Notices,” vol. 27, pp. 87 and 88.) In a letter to the 
“ Astronomical Register,” vol. 5, p. 51, Mr. Knott further states 
that, other things being equal, the separating power of a telescope 
depends solely on its aperture, the diameter of the disc and rings 
varying inversely as the aperture ; in other words, the larger the 
diameter of the object glass the smaller is that of the apparent 
image of the star. 
inent optician Dallmeyer agrees closely with Mr. 
Dawes i in ve separating power of telescopes, which he finds to be 
for 
On the other hand, Mr. Warren de la Rue thinks that focal 
length has something to do with the size of the discs (“ Astro- 
nomical Register,” vol. 5, p. 128). 
Steinheil, the well known German optician, is of opinion 
“ Astronomische Recbricktan ”No. 1525—“Intellectual Observer,” 
vol. 7, p. 480) that it is a known consequence of the diffraction of 
light that the stars appear as discs with measurable diameters. 
ese diameters stand in relation to the intensity of the light, 
and the proportion that exists between the aperture of an object 
glass and its focal length, so that a telescope of 8 inches aperture 
and 12 feet focal length, ‘which could not divide 7° Andromede, 
would necessarily give to the component stars ae dises than 
one of the same aperture and shorter focal lengt 
Dawes controverted this opinion, and ‘Steinheil partly 
accepted his correction, and explained the fact that his 8-inch 
telescope did not separate stars which other instruments of the 
its brilliancy. It has been long known that a central patch to stop - 
out the middle rays has reduced the brilliancy of the stars and 
their discs, and thus increased the separatin: ris - a telescope; 
hence the superiority of a reflector, the small reflector acting as @ 
stop to the central rays. (“Intellectual Observer,” vol 8, p. msi 
It will be seen from the foregoing extracts that t e balance of 
opinion is that the separating power is nesheset ns a function of 
only, but we rt in Sydney refractors up to 11} inches, 
and reflectors to 10} inches acini and experiments could be — 
. by a Section belated ah? 
this i interesting questio’ 
