266 PROCEEDINGS OF SECTION A. 
values of the aberrations of the two surfaces at their respective 
centres of curvature, which determine their actual form, and 
finally indicating the manner in which an error in the curvature 
of one surface may be compensated by an appropriate correction 
in the curvature of the other. 
In diagram below, which shows the arrangement of the mirrors 
under test, A is the primary or conave mirror 4 feet in diameter 
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DIAGRAM SHOWING ARRANGEMENT OF MIRRORS OF THE GREAT MELBOURNE REFLECTOR DURING 
THE PROCESS OF TESTING AT THEIR CENTRE OF CURVATURE. 
and 366: inches focus (approximately). a is the secondary or 
convex mirror 8 inches in diameter, and 74# inches focus. Lis the 
source of light (a kerosene lamp) in front of which is placed a 
small diaphram O, => inches aperture. E is the place of the 
eye situated behind a small vertical knife-edge O', which is capable 
of being moved with a gentle motion across the line of sight. 
The diaphram O is located at the centre of curvature of the mirror 
A, and the mirror a is so placed that its centre of curvature C is 
at approximately the same distance from mirror A as the diaphram 
O. Then the rays of light diverging from the aperture O fall on 
mirror A, by which they are reflected towards the point C ; but 
on encountering the surface a they are reflected back along the 
fainter lines, diverging from C', and fall again slightly displaced 
on mirror A, by which they are again reflected at a point O' near 
the knife edge, from which point the observer views the effect 
produced when the knife edge is being moved across. The form 
of the surfaces is determined by the length of the radius of 
curvature of each of their elementary zones. Wooden diaphrams 
are accordingly placed before mirror A, by which the whole 
reflecting surface can be subdivided into concentric rings 3 inches 
wide, varying in diameter from 45 to 12 inches, one ring at a time 
