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CH. IX] MICRO-PROJECTION WITH LIME LIGHT 287
distance from one another are as for the electric light described above.
As the spot of light must remain in exactly the same place to be received by the small lenses of the projection objective, it is necessary to regulate the hand heliostats oftener than for the magic lantern.
It may also be necessary to make slight corrections in the mirror of the clock-driven heliostat from time to time. The law is : The axial ray must correspond with the optic axis of the apparatus.
§ 421. Use of a water-cell. — The radiant energy of the sun is so great that a water-cell to remove as much of it as possible except the luminous part (§ 844) is as desirable as with the electric light. It is also desirable to have a specimen cooler (fig. 121).
PROJECTION MICROSCOPE WITH THE LIME LIGHT
§ 422. The management of the lime light for the projection microscope is exactly as for the magic lantern (see § 1 63 , 1 64) , only more attention will be necessary to keep the best possible light all the time. The image of the luminous spot should be focused on the hood of the objective as for the electric arc. While there is not so much danger from overheating as with the electric light or sunlight, it is desirable to use a large water-cell. The stage cooler is also an advantage. For the correct form of a condenser see § 363.
As the intrinsic brilliancy of the lime light is less than that of sunlight or the electric light one must not expect so much of it as of them.
§ 423. Other sources of light are insufficient to give good micro-projection except in a very limited degree, and for some special purposes. See under drawing, Ch. X, § 463.
HOME-MADE PROJECTION APPARATUS
§ 424. Projection table. — For all kinds of projection the table should be of convenient height, so that the operator can stand during the exhibition. A height of 100 centimeters (40 inches) is suitable for most persons. The size of the top varies greatly with