Some years later, in 1896, Lord Rayleigh ( 1896) discusses extensively the resolution of microscopes. He considers 1/(2 ω) as the resolution limit.ĭetailed experimental tests of Abbe’s theory including the demonstration of artifacts in the microscopic images are published by J. He finds that for circular apertures sin ω=0.819 λ/ R, where ω denotes the viewing angle of the first bright ring, λ the wavelength of the light used and R the radius of the aperture. in terms of viewing angle and aperture diameter). He uses a slightly different separation criterion and arrives at similar results for resolution as later Abbe and von Helmholtz, which he derives for the case of telescopes (i. In 1869 Émile Verdet ( 1869) seems to be one of the first who explicitly mention that microscopes are limited in their resolution by diffraction g,h. Like Abbe he does not recognize that diffraction effects would remain even with self-luminous objects and would hence limit the resolution.Īlthough the articles from Abbe and von Helmholtz are the first ones dealing in detail with the resolution limitations of microscopes, the effects of diffraction and its implication for resolution were known earlier. He denotes the persistence of diffraction to the remaining phase relationships in the object plane. From his theory he concludes that diffraction effects should then vanish. incoherently) by imaging the light source onto the object. In addition, von Helmholtz tries to illuminate the object in a way that avoids phase relations at different object points (i. In the last paragraph of his article he states that he had finished his work before he became aware of Abbe’s publication and that it seems acceptable for him to publish his findings in addition to Abbe’s work for they contained the mathematical proofs, which were missing in Abbe’s article. In contrast to Abbe, von Helmholtz gives a detailed mathematical derivation of his findings. Only one year after Abbe’s first article about the resolution limit (Abbe 1873) appeared, Hermann von Helmholtz published the same results f (von Helmholtz 1874). Nevertheless, in his article from 1873 (Abbe 1873), he already acknowledges the possibility of new developments that are not covered by his theory and that might enhance the possibilities of optical microscopes beyond the limits that he derived e. As becomes apparent in a later article (Abbe 1880), he did therefore not recognize that the same resolution limits also apply to self-luminous objects d (as used in fluorescence microscopy, which was developed much later). The object diffracts the illuminating light and only if a sufficient number of diffraction orders passes the finite-sized objective, the object can be resolved. Abbe sees the microscopic object as consisting of diffraction gratings. It is interesting to note that Abbe’s 56-page article does not contain any formula in mathematical notation. Abbe does not discuss explicitly the influence of the refractive index in the sample and the immersion medium, though he does consider immersion objectives. Where d min is the minimal resolvable distance, λ the wavelength of the light, and α the half aperture angle of the microscope’s objective c it is left open whether λ refers to the wavelength in the immersion medium or in air.
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