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By Mortimer Abramowitz

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This kind of illumination, originally called critical illumination, frequently suffered from unevenness because of the limited size of the light source or because the image of the light source was obtrusively superimposed on the image of the specimen. In the early 1900’s, August Koehler and others developed a procedure for providing bright, even illumination superbly suited for both microscopy and photomicrography. Koehler illumination is the method of choice in all modern microscopy and photomicrography and imaging for transmitted as well as reflected light techniques.

In b, the air space is replaced by oil of the same refractive index as glass. The rays now pass straight through without deviation so that rays 1, 2, 3, and 4 can enter the objective. A. is thus increased by the factor n, the refractive index of oil. 515 for oil. · Since µ cannot exceed 90°, the sine of µ must be 1 or less. A. 95. · Increasing the angular aperture of an objective increases µ and thus increases sine µ and thus increases numerical aperture. 4. PAGE 34 / THE SIGNIFICANT DETAILS Fig 41.

Image reduced in size. CASE 3 The object is brought to twice the focal distance in front of the lens. The image is now two focal lengths behind the lens. It is the same size as the object; it is real and inverted. This is the case for so-called 1 to 1 photography. Fig 28. Object at twice the focal length. Image at full size. PAGE 24 / THE SIGNIFICANT DETAILS CASE 4 The object is situated between one and two focal lengths in front of the lens. Now the image is still further away from the back of the lens.

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Microscope - Basic and Beyond by Mortimer Abramowitz

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