SPECIAL METHOD OF PHOTOGRAPHING OPAQUE OBJECTS.
So far only transparent objects, which are photographed by transmitted light, have been dealt with ; opaque objects, such as etched metals, etc., require to.be illuminated by reflected light, and neces sarily the apparatus is slightly modified. The usual plan of illuminating by reflected light is to place the lamp at the side of the microscope and to concentrate the light upon the object by means of the bull's-eye condenser ; but the illumination is then all on one side, and raised objects, 3 x 2 in. divided by a transparent line will be obtained on the one plate. In de termining the intensity of the light and in photographing bacteria and other minute objects this will be found a saving in plates, and the work will be quite as satisfactory as if it were done upon whole quarter plates. When a whole quarter plate has to be exposed the hinged flap of wood is allowed to drop outwards, and the dark slide can then be used to its full extent. An arrangement on the same principle can be used to adapt any size of camera for photomicrographic work.
such as microscopic corals, etc., arc in deep shadow on the side opposite the light. To obtain a good light on both sides of the object the parabolic side re flector (Fig. 737) is placed on the stage of
the microscope, and the light from the condenser is reflected from it. This method of illuminating is possible only with low power objectives, as thqy have a considerable focal length ; with high power objectives the focus is so very short that the objective casts a shadow on the object, and it is impossible to get any light on the object in this way. The vertical illuminator of Zeiss (Fig. however, be used for high powers ; it con sists of a short tube with a circular open ing in the side ; in front of this opening is a small prism, which may be rotated by a milled head outside the tube. The illu minator is screwed upon the body tube of the microscope in front of the objective. A beam of light is projected through the opening of the illuminator, and is re flected from the surface of the prism through the objective on to the object below, the image of the object being visible through the open half of the objective. A vertical illuminator may also be ob tained, in which the light is reflected from the surface of a thin glass circle fixed at an angle of 45', through which the object is easily visible (Fig. 739).