Different samples require different microscopes. This rule refers to the fact that an opaque, bulky sample with a reflective surface needs another treatment than a transparent, unstained smear from the cavitas oris. The microscope stand offers the necessary space for a correct positioning of the sample and all options for the appropriate illumination method.
The microscope objective is an even more specific item. Here we talk about the required resolution power (= numerical aperture), but also about cover glass correction, immersion method and working distance.
5. Objectives for Polarization Microscopes just have one purpose. Here we do not talk about color fidelity (in any case Plan Apos will be best) or transmission rates (Fluorite objectives with a reasonable quantity of glass built in are mostly recommended). Here we talk about strain-free glass elements mounted without tension within the objective. Once this target is achieved, the purpose of these objectives is fulfilled: maximum extinction when polarizer and analyzer are crossed.
6. Following another international coding, Phase Contrast objectives are marked with a green inscription. Phase contrast is a contrast method for transmitted light, but the respective objectives can also be used in bright field. A slight reduction of image quality is system immanent.
7. For metallurgical applications, dark field (DF) in incident light is quite common. The necessary objectives have to have a larger diameter to incorporate the DF illumination ring: a mirror system built around the centrally positioned bright field objective. Analog to the situation in transmitted light, a central stop in the Epi illuminator (some people call this device Epi condenser!) stops the direct light entering the objective. So the periphery of such an objective works as an illuminator, the central part as an imager.
8. For Inverted Microscopes in bio/medical applications, especially higher magnifications need a Long-Working-Distance construction. The LWD feature works of course at the expense of resolution power.
The idea behind such a construction is quite clear. The samples for an inverted microscope are positioned in a petri dish, a flask or any other “high volume” vessel. Adherend cells at the bottom of the vessel have to be treated with an objective of 1.1mm cover slip correction (in this case the bottom of the vessel is equivalent to the cover slip). Floating cells or water samples from a pond need the additional LWD feature. Only this construction allows a focusing “through” the sample, inspecting the complete fluid layer. A short look on motorized inverted microscopes gives us an interesting hint: Before changing the objective by rotating the nosepiece, first drive down the revolving nosepiece. This will avoid scratches on the front lens. Parfocality of the objectives in this case is less relevant.
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