DR. OLIVER LODGE ON ABERRATION PROBLEMS. 
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front mirror; and the width of the beam may be as much as inch. The pair of 
plates between which the light has to go are, however, only 1 inch apart, so this 
limits the effective aperture in a direction perpendicular to the plane of reflexion. 
A narrower beam may be sent more times round, by setting back the fourth mirror 
the proper amount. 
Fig. 8. 
Diagram showing arrangement of mii’rors for the usual case of beam going three times round. Inter¬ 
fering rays travel parallel but not identical paths, and re-unite at a point different from that at 
which they split. The greatest possible size of beam, with a 4-inch fourth mirror, is represented. 
The centre of the semi-transparent glass, f.e., the intersection of telescope and collimator axes, has 
to be 2 inches behind square formed by the three mirror faces. The fourth or extra mirror has to 
be 1 inch in front of the same square. 
Whir ting Machine. 
34. For the pair of plates I use a couple of steel circular-saw disks, one yard in 
diameter, of best hammered steel, and bevelled down somewhat from centre to edges, 
braced up at centre with wrought-iron cheeks and bolts. The plates were specially 
made by Seebohm and Dieckstahl, of Sheffield, and are stated to be able to stand 
sixty-seven tons to the square inch. 
At this strength they would fly to pieces at 8800 revolutions per minute, supposing 
