July 14, 192 1] 



NATURE 



035 



An Interferometer for Testing Camera Lenses.^ 



INTERFEROiMETERS for the testing and correc- 

 tion of prisms and of lenses (for axial pencils) have 

 been described in the Philosophical Magazine 

 (vol. XXXV., January, 1918, p. 49). In its simplest 



Figs, i an 1 2. — Plan and sidcelevation of lens interferometer. 



form the instrument resembles the well-known Michel- 

 son interferometer, the essential optical difference being 

 that the two interfering beams of light are brought 

 to a focus at the eye of the observer. The principles 

 of the prism interferometer have been applied to 



Fig. I, and a side elevation in Fig. 2. Light from 

 a suitable source is reflected by a mirror 10 into 

 the interferometer. A convex spherical mirror 001 

 is so disposed that its centre of curvature coincides 

 with the focus of the lens 14 which 

 is under test. In these circum- 

 stances, a beam the wave-front of 

 which is a plane perpendicular to 

 the axis of the lens will, after pas- 

 sage through the lens, be reflected 

 back on its own path by the convex 

 mirror, and if the lens be free" from 

 spherical aberration the reflected 

 beam will, after passage through 

 the lens, once more have a plane 

 wave-front. If it has not, then the 

 departure from planeness will pro- 

 duce interference bands which form 

 a contour map of the corrections 

 which will have to be applied to the 

 lens to make its performance per- 

 fect. 



An apparatus which will test for 

 axial pencils only is, of course, of 

 little use for testing camera lenses. 

 The modifications essential for the 

 latter purpose are (i) means of 

 rotating the lens about a line at 

 right angles to the axis and passing 

 through the second principal point, 

 and (2j mechanism whereby, simul- 

 taneously with the above rotation 

 of the lens, the convex back-reflect- 

 ing mirror is automatically moved 

 away from the lens jn such a way 

 that its centre of curvature always 

 falls on the plane, perpendicular to 

 the axis of the lens, on which the 

 lens is desired to form its image. 

 The rotation of the lens car- 

 riage is effected by means of a bar 105 parallel to 

 the axis of the lens and extending to the outer edge 

 of the interferometer. The second requirement is 

 fulfilled by a flexible connection being led from the 

 carriage on which the mirror is adjustably mounted 



Fig. 3. — Interferograms of a photographic lens for axial and oblique beams. 



photographic lens testing in the camera lens inter- 

 ferometer recently constructed by Messrs. Adam 

 Hilger, Ltd, A plan of the instrument is shown in 



1 Abstract of a pap-r read before the Optical Society on April 14 by 

 7. Twyman. 



NO. 2698, VOL. 107] 



and over a pulley to a weight, and by there being 

 upon the carriage a roller which by the action of 

 the weight is retained in contact with a cross-bar 

 mounted on the axial bar 105 and at right angles 

 to it. 



