. The microscope; a simple handbook. Microscopes. 34 . 'THE MICROSCOPE. Fig. 23.âNo. 3295, High-power Dark- ground Illuminator. shows the optical portion of this illuminator. The light from the mirror being thrown upon the under-surfaoe of the glass reflector, the light is reflected by two curved surfaces so that a ring of light is focussed to a point upon the object at a very oblique angle, as shown by the white portion of the diagram. The whole of this light is so oblique that it mil all be totally reflected inside the glass and will not emerge from the illuminator unless the latter is broug
. The microscope; a simple handbook. Microscopes. 34 . 'THE MICROSCOPE. Fig. 23.âNo. 3295, High-power Dark- ground Illuminator. shows the optical portion of this illuminator. The light from the mirror being thrown upon the under-surfaoe of the glass reflector, the light is reflected by two curved surfaces so that a ring of light is focussed to a point upon the object at a very oblique angle, as shown by the white portion of the diagram. The whole of this light is so oblique that it mil all be totally reflected inside the glass and will not emerge from the illuminator unless the latter is brought into immersion contact with the under-surface of the slide by placing a drop of cedar-wood oil be- tween the top of the Illuminator and the slide. It must be used in immersion contact with the slide in the same way that an oil-immersion object glass is used in inunersion contact with the cover glass. With this illuminator any dry lens or an immersion lens with an aperture of less than 1 can be used, and no direct light, but only that reflected by the Tjsoofoii- object, enters the microscope (see Fig. 24). A special oil- ibf^or'"" immersion 1/8-inch (3-mm.) focus, with an aperture of "95 , glares with jg made for work with this illuminator; or an immersion lens " ^'"" â with a larger aperture can be used if it be stopped down by means of a small diaphragm placed behind the back lens of the object glass. In the latter case the object glass must be stopped down to a considerably smafler aperture than 1 , because the stop cannot be placed in the best position, which is between the lenses, themselves, and with a stop behind the back lens a certain amount of direct light is not properly excluded by a stop of the theoretical size, because it is not in the correct position. There is a peculiarity in dark-ground illumination. The object must be exactly at the crossing point of the beams of lightâ that is, in its focusâor it will not be i
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Keywords: ., bookcentury1900, bookdecade1920, booksubjectmicroscopes, bookyear
