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CH. I] CENTERING THE MAGIC -LANTERN 43
With the two-lens condenser used for lantern slides the lamp is usually closer than the principal focal distance of the first lens, this makes the beam between the lenses diverging, hence it is best to have the two lenses as close together as possible to avoid loss of light (fig. i).
With this condenser and diverging light between the lenses the only rule that can be given is to adjust the distance between the lamp and the condenser until the best light is obtained on the screen. If this brings the crater of the arc lamp within 8 to 10 cm. (4 in.) of the first lens, then it will be necessary to substitute longer focus lenses for either the first or the second condenser lens or for both. In general, the first lens should be of about 15 cm. (6 in.) focus and the second lens should have a somewhat shorter focal length than the projection objective. For example, if the projection objective is of 38 cm. (15 in.) focus, the second lens of the condenser in the two-lens form should be of about 25—30 cm. (10—12 in.) focus. This will bring the diverging cone to a focus near the center
of the objective. •
§ 56. Condenser and projection objective. — If the projection objective and the condenser are properly proportioned the condenser will focus the light near the center of the projection objective when the lantern slide is in focus on the screen (fig. 1,2).
If the condenser is of so short a focus that the light from the condenser comes to a focus before reaching the objective the field is restricted and bordered by a red margin (fig. 29).
If, on the other hand, the condenser is of too long a focus for the objective the light will not come to a focus by the time it reaches the center of the objective (fig. 28). In this case the field will be restricted and bordered by blue.
OPTICAL TEST FOR CENTERING
§ 57. Optical test for centering the radiant and the condenser.
— If these are properly centered along one line, and the two are separated a considerable distance when the lamp is burning, the light spot on the face of the condenser looking toward the objective