INCANDESCENT GASLIGHT.
For use in photomicrography an incan descent gas burner may be supported on a stand at the proper height so that the 3 in. diameter and 10 in. focus will be found to yield a very intense illumination. The flame of an oil lamp is much 3iellower than the illuminants already considered, and therefore its actinic value is not -in proportion to its intensity ; in conse quence the exposures required are a little longer than would be the case with a whiter light of the same intensity. With an ordinary oil lamp, a bull's-eye con brightest part of the mantle may be used for illuminating purposes ; or the burner may be mounted on an arm which can be made to slide on a metal rod and clamped in the proper position. If the old form of burner is used the chimney should be of mica, and the mantle should be sup ported upon a platinum wire stretching across the chimney at the proper height ; the usual clay supporting rod cannot be used because it produces a dark shadow in the centre of the field of view. In the
newer Kern burners the support is a metal wire on the outside of the mantle and no chimney is required. Fig. 681 shows an incandescent lamp sliding on a rod as above described. It is fitted with a metal hood which cuts off all extraneous light, and may also be provided with an iris diaphragm, which, when partially closed, eliminates the structure of the mantle. if no diaphragm is used, then a slip of ground glass must be placed be tween the burner and the microscope, or the structure of the mantle would ap pear on the focussing screen and would also be seen in the finished photograph. The incandescent gaslight is better than the light of an oil lamp, being more in tense and also much richer in actinic rays.