CRITICAL ILLUMINATION In microscopy, critical illumination is obtained by arranging the illuminant, mirror, and substage condenser in such a manner that the image of the lamp flame is seen in the centre of the field when looking through the microscope. Critical illumination is not practicable with low powers, but it is essential when obtaining the finest definition a lens which will give with high powers, and more especially when using the immersion lenses. Recent experiments have shown that critical illumination has been obtained when the back lens, on looking down the tube without an eyepiece, is just filled with light. Daylight can not be used for critical work. The Nernst lamp is a convenient light for photo-micrography, but for visual purposes an oil-lamp with f-in. wick gives excellent results. The lamp, with the flame edgeways to the microscope, should be placed with the wick about 9 in. in front of the mirror, which must have the plane side turned to the light. The mirror is placed so that the
light is reflected in a direct line through the substage condenser and objective. A diatom is placed on the stage and the substage condenser racked up and down till the image of the flame is seen in the centre of the field. If the eyepiece is now removed, the back lens should (providing the substage condenser is of sufficiently high aperture) be filled, or nearly filled, with light. That portion of the field of view only which contains the image of the lamp flame gives critical definition.
If the flame image is a disturbing factor to the work in hand, it can be removed by slightly lowering the condenser, but the definition suffers. Critical illumination is necessary only for powers of f in. and upwards. Low powers should be worked without the substage condenser, and with the concave surface of the mirror turned to the light.