Celestron Advanced Stereo Microscope Microscope Basics - Page 17

the eyepiece (tube) lens. The tube length is a fixed measurement in millimeters of the distance from the objective shoulder to the seat of the eyepiece which is typically DIN 160mm or 170mm (except on introductory microscopes) where the focused image forms. This distance governs the interchangeability of optical components. Tube Lens At the bottom of the eyepiece tubes is a tube lens. Its function is to gather the parallel rays of light projected by the objective lens and bring those rays to focus at the plane of the fixed diaphragm of the eyepiece. On some microscopes, the tube lens is built into the body of the microscope itself. Stage The platform beneath the objectives on which the slide or object to be observed is placed is called a stage. It has a smooth, flat surface and can be rectangular or circular. On most compound microscopes, the stage moves up and down and the nosepiece is stationary but on some microscopes just the reverse takes place. The stage has an opening for passing light. The simple type of stage is called a plain stage and the more sophisticated stage is called a mechanical stage. The more sophisticated and expensive mechanical stage makes it much easier to center specimen slides (standard slide size is 1"x 3") with precise movements in two axes with knobs which is critical at high powers (400x or more) where slight movements of the slide by hand moves the slide too much or out of the field of view. A vernier type scale on the stage allows for making measurements of the specimen. The "X" axis moves a slide forward and back (north/south) and the "Y" axis moves the slide side-to-side (east/west or left/right). Plain Stage Mechanical Stage Stage Plate A stage plate is used with stereo microscopes and this plate fits in over the lower illumination in a circular hole. It normally is a frosted glass plate or a dual sided (white and black) plastic plate which can be used for improving contrast on various specimens to be viewed with top illumination. The working distance of a stage is the vertical area that is available to place specimens for examination. Terms Associated with Microscopes Magnification (power) - the magnification of a microscope is determined by multiplying the power of the eyepiece by the power of the objective lens being used. As an example, a 40x objective lens times 10x eyepiece = 400x. Another way to look at this is that the magnification is the number of times the image is larger than the object would appear to the unaided (naked) eye. Lower powers allow for brighter, sharper images combined with a wide field of view. Higher powers, often for examining slide specimens, present larger but dimmer images with a narrower field of view. When observing, always start with the lowest power on your microscope and progress to higher powers. High powers are useless unless the resolution is good. When the magnification is increased but the detail is not increased (due to the limitation of the resolution), it is called "empty magnification". For a given magnification, always use the higher magnification objective when given a choice rather than using a lower power objective with a high powered eyepiece as the results will always be better. For example, using a 40x objective and 10x eyepiece =400x will give higher quality views than using a 20x objective and 20x eyepiece =400x. Field of View - the field of view is the diameter of the circle of light that you see when looking into a microscope and it is measured in millimeters. The lowest powers have the widest field of view. As you increase power, the field of view gets smaller. Some wide angle or super wide angle eyepieces increase the field of view over standard eyepieces. 17