TESTING THE CENTERING. 171 



before the unscrewing " a proposition the theoretical accuracy of 

 which is unquestionable. Since that date we have become con- 

 vinced that the process amounts to nothing more than a testing of 

 the mountings. The errors of centering, which still occur in the 

 mounting of the lenses, even in the best workshops, are so small 

 in comparison with the excentricity of the screw-thread, that they 

 play only an insignificant part in the movements of the image. 

 If then the lenses, independent of the mountings, are to be tested 

 for centering, their unscrewing is evidently detrimental to the 

 purpose. The only efficient process of testing the centering is 

 that applied in practical Optics as follows : 



The metal cell of the back combination, which we will assume 

 to be concentric, is attached to a lathe, so that its own axis 

 approximately coincides with the revolving axis. The accurate 

 testing of this coincidence may be effected by means of a so-called 

 "lever of contact," the shorter arm of which rests, during the 

 revolution, upon the free surface of the cell, while the longer 

 one projects freely backwards, and in case of inaccurate centering 

 exhibits the movements of the short arm on an enlarged scale. 

 When the cell is adjusted satisfactorily concentric, the lens is 

 temporarily inserted and moved variously, until the errors of 

 centering are eliminated as completely as possible. These errors 

 are shown by the motion of the small reflected image which the 

 lens forms of a window-frame or lamp-flame. The optician 

 observes this image with a magnifying-glass, and moves the lens 

 in its cell until the image remains perfectly still during the 

 revolution. Then the projecting metal edge of the cell is burred 

 upon the lens, by which it is permanently mounted. The second 

 and third combinations are tested similarly after the cells have 

 been adjusted. 



In the testing of completed objectives, we screw the brass- work 

 on the lathe, and commence with the back combination alone, 

 and so on, adding the others. When in each case the reflected 

 image remains motionless during the revolution, the centering is 

 practically perfect. 



