784 Mr. F. Twym.an on an Interferometer 



on a frame, 141, adjustable on the plate 220 by the screw 

 and milled head, 239 (fig. 8), along the optical axis. A 

 pivot, 222, slides in a slot and can be clamped to the plate 

 220 in any desired position by the clamp 223. The pivot 

 works in a bearing, 162, which is also the bearing for the 

 roller 163. The plate 142 rests on the carriage 140, on the 

 underside of which the bearing 162 is clamped by the bolt 

 165 and nut 166. A long slot enables the bearing 162 with 

 the roller 163 to be clamped in any position on the carriage. 



These adjustments enable the centre of curvature of the 

 mirror to be brought exactly on the axis of the roller and on the 

 optical axis of the lens under test. The roller 163 is main- 

 tained in contact with the crossbar 160 by means of a flexible 

 connexion to a weight (not shown on the drawings). The 

 carriage 140 slides parallel to the optical axis on the ridged 

 bed, 101; thus when the lens 14 is rotated hj the crossbar 

 160 the roller keeping contact with the crossbar auto- 

 matically shifts the carriage in such a way that the mirror 

 centre of curvature is always in the plane in which it is 

 desired that the lens should focus its image. The vernier 

 123 (fig. 10) measures the distance from the centre of the 

 roller in the axial position to the axis of rotation of the lens ; 

 thus when all adjustments have been made this vernier gives 

 the focal length of the lens to an accuracy of about 0*001 inch. 

 In fig. 3 the lens, 14, and mirror, 001, with focal plane, 22, 

 are shown in full lines, and the corresponding position for 

 an oblique pencil in dotted lines. 



The mirror, 007 (fig. 5), is mounted on a carriage moving 

 on grooves, 72, and actuated by means of cords, 73. 009 is 

 a lens through which the recombined beams from the mirrors 

 007 and 001 pass, being thereby converged on to the ob- 

 server's eye. 



Adjustment of the Apparatus, and setting up a lens for test. 



It will be sufficiently obvious to members of this Society, 

 accustomed as they are to make or use optical instruments, 

 that the geometrical conditions implied in the above description 

 must be attained with considerable accuracy if the apparatus 

 is to form a reliable test of performance of a camera lens. 

 Furthermore, it must be possible to perform all the necessary 

 adjustments preliminary to a test in a simple, systematic, and 

 speedy manner if the apparatus is to form a useful tool in 

 connexion with the manufacture of camera lenses. A routine 

 has been worked out whereby an observer can satisfy himself 

 that the geometric conditions are successfully fulfilled, and a 



