Mode of Formation of the Image in the Microscope

MODE OF FORMATION OF THE IMAGE IN THE MICROSCOPE We may take it that the way in which a lens forms the image of a luminous object is understood and generally accepted. The formation of the image of an illuminated non-luminous object requires consideration, however, since the light reaching the lens, whether reflected from, refracted through, or passing round the object, originates from an independent source of illumination, and the object is only "seen" as a result of the way in which it modifies the light which falls upon it. As regards large objects common experience shows that the way in which their images are formed, as in the act of "seeing," can be considered as substan tially the same as that in which the images of luminous objects are formed, with the exception that a non-luminous object may, in certain circumstances, be "seen" merely as an opaque mass against a luminous or brightly illuminated background.

At one time it was commonly accepted that an object is seen in the microscope in essentially the same way as larger objects are ordinarily seen, and that the processes involved in forming the microscopic image of a small non-luminous object are the same as those involved in forming the image of a self-luminous body. These were the views put forward by such students as Airy, Arago, Foucault, J. F. W. Herschel, and Helmholtz. When light falls on a very small object, however, diffraction effects, which are not usually observed when the object is large, come into prominence. Abbe made a study of these effects and, as a result of his observations, put forward a new theory of the mode of formation of the image in the microscope. This theory is given in most treatises on the microscope and microscopy and need only be briefly outlined here.

Abbe's Diffraction

white light from a distant small source falls perpendicularly on to a diffraction grating, such as could be made by ruling fine lines at regular small inter vals through a silver film deposited on glass, much of the light passes straight through the grating in the ordinary way. On either side of the direct beam, however, beams of coloured light will emerge, the directions of these beams being inclined at various angles to the direct central beam. An object-glass of

suitable aperture placed close up to the grating would receive the direct beam and certain of the "diffracted" beams, the number of diffracted beams admitted by the object-glass being dependent on the spacing of the lines in the grating and on the numerical aperture of the object-glass. Each of the beams admitted would give rise to a small image of the source, so that immediately behind the object-glass there would be a central white image of the source with elongated coloured images lying on either side of this. In any particular part of one of these images the light of any one colour vibrates in a manner which bears very precise relationships to the vibrations of light of the same colour in corresponding parts of the other images. As a result, interference fringes will be produced in the body tube of the microscope, the distribution of brightness seen in any particular plane being determined by the number and position of the images formed behind the object-glass.

According to Abbe, the "image" seen on looking through the eyepiece is simply the interference pattern in the focal plane of the eyepiece. In so far as this pattern is determined by, and therefore characteristic of, the structure of the object, it may be considered to be an "image" of the object, but it cannot be claimed that the form of this "image" bears any true resemblance to the object. Abbe based these views on observations made with gratings illuminated by a parallel or nearly parallel pencil of rays, using object-glasses behind which he could place small dia phragms so that the different diffraction images of the source of light could be cut out or admitted at will. He claimed that the results he obtained in this way are of general applicability, and that the images of non-luminous objects are formed exclusively as the result of interference of the type described, no matter what the structure of the object and irrespective of the way in which the object is illuminated.