Ch. VII] PROJECTION OF IMAGES OF OPAQUE OBJECTS 179 This «apparatus is-designed for all kinds of projection, and with the objects cither in a vertical or in a horizontal position. When the object is in a vertical position the illuminating device (arc lamp with parabolic reflector) sends the light horizontally through the specimen, apparatus and to the screen as would be the case in the figure here shown. If the object is in a horizontal position the lamp and reflector remain in a horizontal position and the light is reflected by a mirror upon the opaque object ; or for vertical opaque objects the radiant is turned sidewise. For transparencies in a horizontal position the lamp and reflector are lowered to the level of one of the mirrors below, and this mirror reflects the horizontal beam up through the transparent object whence it passes to the projector and the screen. The entire apparatus is covered by a dark curtain (compare fig. 95). Use of Opaque Projection for Exhibitions and for Demonstrations § 283. Testing the lantern.—The directions given in Chapter I, §26 are applicable here. § 284. Size of objects for opaque projection.—The size of object which can be shown with an opaque projector varies greatly. The smallest size is usually larger than a lantern slide. The lan¬ tern-slide opening is rarely greater than 6.5 x 7.5 cm. (2.6 x 3 in.), while the smallest picture usually shown in the opaque lantern is rarely less than postal card size (8 x 12.5 cm., 3x5 in.). From this minimum the size ranges all the way up to 50 cm. (20 in.) square. Of course the radiant and condenser must vary accordingly (see fig. 107). § 285. Objects for opaque projection.—The best of all are dull white objects, like marble figures, or black print on white paper, pictures in black and white. Colored pictures in which the bright colors of the spectrum like red, yellow and green, arc predominant, give good images. Metallic objects with polished surfaces give good images. Among these the works of a watch or small clock show well; also coins and medals. Bright metallic objects show best on a dark ground. Objects and pictures which are very light-absorbing naturally will not give good screen images, no matter how brilliant the light or good the apparatus. If the outlines of such objects are what is