THE TESTING OF MICROSCOPE OBJECTIVES AND 

 MICROSCOPES BY INTERFEROMETRY. 



By F. Twyman. 



My firm has no commercial interest in microscopes, and so far 1 

 have not succeeded in interesting any microscope makers in the methods 

 of test I shall describe. We have, therefore, not done much more on the 

 subject than to test a few microscope objectives, and these, although 

 by makers of repute, not of high power. They show aberrations of 

 wave surface not exceeding about 5 wave-length for monoclrromatic 

 light (wave-length 5461). It will be remembered that if aberrations 

 do not exceed ^ wave-length, the resolving power of an optical system 

 is practically perfect. This was found by Rayleigh to be the case in 

 certain cases calculated by him, and general experience shows it to 

 be a sound rule. 



The interferometer used for microscope lenses was a side issue 

 in the development of other forms.* 



An image ' of a monochromatic light source is thrown on a 

 diaphragm which has a small hole. The light passes to a half- 

 silvered mirror (Figure 1). A portion of the light is reflected 

 from there to a concave mirror so situated that the diaphragm is 

 approximately at its centre. From the concave mirror the rays are 

 reflected, and a portion of the light passes through the half -silvered 

 mirror, and is focussed on the eye of the observer. The light which, 

 on meeting the half-silvered mirror passes through it, proceeds 

 through a compensating plate as in the Michelson Interferometer; 

 then through the objective under test. The rays pass on through the 

 image, and are reflected back on their own path from a concave mirror. 

 Eventually the two beams of light combine at the surface of the 

 half -silvered mirror, and pass on together to the eye. In these 

 circumstances interference effects are observed which appear to the 

 observer as if located on the back len? of the objective under test, 

 and which represent a contour map to a scale of half wave-lengths 

 of the aberrations of wave surface produced by the objective under test. 



If desired, an entire optical instrument, such as a microscope, can 

 be tested, in which case the arrangement is as shown in Figure 2. 



* Described by the present writer in the Phil. Mag., Vol. XXXV., 

 January, IQ18, " Interferometers for the experimental study of optical systems 

 from the point of view of the wave theory." 



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