729. Plates

Plates. While, ordinarily, we would recommend the use of orthochromatic plates for all microscopic work, yet for this class of work and for first experiments any ordinary plate will answer - one having a slow emulsion preferred. The object of using orthochromatic plates is to preserve the relative color values that exist in different objects to be photographed. Wherever such values exist, either an orthochromatic, or even a trichromatic or panchromatic plate, which is sensitive to all colors, should be used; but for plain work void of colors, the ordinary or double-coated non-halation plate will answer every purpose.

730. Exposure

Exposure. With the camera and plate-holder in position, the next in order would be the making of the exposure. The cheaper photo-microscopic cameras not being fitted with a shutter, some means must be employed whereby the illumination may be shut off from the object being photographed. A very simple arrangement will be found in the use of an ordinary cardboard, say 10 x 12 inches, made V-shape so that it will stand erect. This can be accomplished by bending the cardboard in the middle. This will hold the card in position and permit of it standing without any support. When ready to make the exposure, place the cardboard before the stage and in the path of the light, excluding all light from the object on the slide. You are then ready to make the exposure, the length of which depends upon the density of the object being photographed and the strength of the illumination.

731. With the fly-wing specimen if daylight is used you will require about five seconds exposure; with Wels-bach light, thirty seconds. Now, with the shutter before the instrument, excluding all direct rays of light from the object, very carefully withdraw the slide on the lower section of your double plate-holder. After withdrawing the slide, wait a moment until all vibration has ceased, then carefully withdraw the cardboard shutter from in front of the stage for the necessary length of exposure and again replace, after which insert the slide and the exposure is completed.

732. Use Of Welsbach Gaslight

Use Of Welsbach Gaslight. In Fig. 1 of Illustration No. 124 we show the use of the Welsbach light in connection with the bull's-eye condenser, for photographing the same object. While in this illustration we show a high-grade microscope, yet the same arrangement will answer with a cheaper instrument, and while gaslight illumination may be employed without the bull's-eye condenser, yet this instrument will add materially to the evenness and strength of the illuminant. By its use you can better concentrate the light upon the object. None of the accessories appearing in this illustration, with the exception of the light and the bull's-eye condenser, are to be used in this instance.

733. You will observe that the bull's-eye condenser is arranged on a perfect line between the light and the mirror of the microscope. The condenser is tilted at an angle to concentrate the light from the lamp upon the mirror. This 'light is collected on the mirror and, in turn, is reflected upward through the slide on the stage. With this accomplished, the remaining manipulation, such as focusing of the image upon the ground-glass, etc., is exactly the same as for daylight work. The only difference between the use of artificial light and daylight lies in the exposure. Owing to the fact that artificial light is less actinic, it requires a longer exposure. Usually the exposure with a Welsbach light, where the bull's-eye condenser is employed, will require double the exposure necessary for daylight work.