for Testing Camera Lenses. 



779 



partly, then wholly, to the next contour line; and so on. 

 The marking out of the prism surface can be done while 



observing. 



Fig. 2. 





^/ 



\ 



/ ^v 





. / 



/ 



In the lens interferometer all parts are left as in the prism 

 instrument except that the mirror F (fig. 1) is removed and 

 replaced by the lens under test and a convex mirror, the 

 latter being so disposed that its centre of curvature coincides 

 with the focus of the lens. Under these circumstances a 

 beam whose wave-front is a plane perpendicular to the 

 axis of the lens will after passage 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, ODce more have a plane wave- 

 front. If it has not, then the departure from planeness will 

 produce interference bands which form a contour map of the 

 corrections which will have to be applied to the lens to make 

 its performance perfect. 



The Camera Tens Interferometer. 



An apparatus which will test for axial pencils only is of 

 course of little use for testing camera lenses. The modifi- 

 cations essential for the latter purpose are : 



1. Means of rotating the lens about a line at right angles 

 to the axis and passing through the second principal point. 



2. Mechanism whereby, simultaneously with the above 

 rotation of the lens, the convex back-reflecting mirror is 

 automatically moved away from the lens in such a way that 

 its centre of curvature always falls on the plane, perpen- 

 dicular to the axis of the lens, on which the lens is desired 

 to form its image. 



3 F2 



