The microscope and some hints on how to use it / E. Leitz.

  • Ernst Leitz (Firm)
Date:
[1910]
Catalogue details

Licence: In copyright

Credit: The microscope and some hints on how to use it / E. Leitz. Source: Wellcome Collection.

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    which we had obtained for the numerical aperture a. We may accordingly write A « = — e From this it follows that to be able to recognise, or resolve, with light of wave-length a the structural outlines of a grating having a constant of diffraction e we require an objective having an aperture of at least. An aperture exceeding the limit which results from the appli- cation of formula (2) enables an objective to take in diffracted pencils of the second, third, and even higher orders. In the actual mi- croscopic image this shows itself by the increased distinctness with which the grating appears to the eye, in that the definition, that is the transition from the dark bars to the bright gaps becomes more and more clearly marked in proportion as the number of diffracted rays which are enabled to enter the objective is augmented. Assuming that the grating is contained in air, and putting n = 1, then equation (1) takes the form sin c/2 - — (3) e Equating the expression on the left of the equations (3) and (1), as obviously w^e may do, w^e find ni sin ci\ sin c<’2 (4) and hence c/i < c/2 (5) From the figures it will be seen that the diffracted pencils are ai'ranged symmetrically on either side of the axis A B in the form of a fan. From equation (5) wx learn that when the grating is enclosed in a denser medium the fan-like system of diffracted pen- cils will close up more and more as the refractive index of the medium increases. This provides the means of causing a larger number of pencils to enter the objective, so as to increase its resolving power. 4Tis fact points at once to the significant influence w'hich the moun- ting medium exercises upon the resolution of finely maiked objects. Supposing that the constant of the grating is so small that the evaluation of the expression furnishes a greater numerical aperture a than that realised in any available objective, then it follow^s from what has been showm above that none but the direct pencil 0 D can enter the objective, and hence the microscope wdll fail to disclose the structural details of the grating. To achieve the desired result in this case w'e may select an extra-axial pencil on the illuminated disc. In practice this is done by displacing the small opening of the iris-diaphragm laterally with
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