for Testing' Camera Lenses. 785 



systematic method of setting the lens and apparatus has been 

 devised, enabling a lens to be mounted on the apparatus and 

 completely tested for five or six obliquities within a quarter 

 of an hour. 



Not to waste space in detail which is not of very general 

 interest, the nature of the adjustments requiring to be made 

 with accuracy will alone be given here. 



(a) The surface of the lens-mount, against which the flange - 

 of the camera lens screws, must Le parallel to the working 

 edge of the crossbar. 



(b) The axis of the collimator must be parallel to the ways, 

 101 (figs. 8 and 9), which bear the mirror carriage. 



(c) The mirrors 007 and 008 (fig. 5) must' be so set that 

 the beam of light from the collimator reflected from 008 

 to 007, and back again, will form an image of the aperture 

 in diaphragm 12 (fig. (3) on that aperture itself. 



(d) The convex mirror must have its centre of curvature 

 on the axis S (fig. 8) of the roller. 



(e) The camera lens under test must have its second nodal 

 point on its axis of rotation. 



(/) The centre of curvature of the convex mirror must 

 coincide with the focus of the lens under test. 



Observation of the Aberrations. 



The apparatus measures the degree to which the wave- 

 front, impressed by the lens on light from a distant point 

 source, differs from a spherical wave-front. The indications 

 are given in aberrations of wave-front to a scale of wave- 

 lengths, the aberration shown being in every case twice that 

 present in the once transmitted beam which normally forms 

 the image of a distant point. The form in which the indi- 

 cations are presented is that of a series of interference fringes 

 which are lines of equal aberration of wave-front. Otherwise 

 expressed, the interference fringes form a contour map of the 

 wave-front with reference to a sphere. 



The aberrations of a centred optical system are, chromatic 

 aberration apart, of the following forms : — 



Spherical aberration in axis, 



Coma, 



Astigmatism, 



Curvature of image, 



Distortion, 



and are characterized as follows, quoting E. T. Whittaker*. 



* ' The Theory of Optical Instruments.' Cambridge University Tress. 

 1907. 



