648 DARK GROUND ILLUMINATION |Ch. XV (3) Diaphragms to shut off the direct light of the lantern. These may be simply sheets of tin. (4) Glass cells with parallel faces. § 914. Method.—Light from the arc L, is ren¬ dered nearly parallel by the lantern condenser Ci. The diaphragm Di, cuts off the lower half of this beam, the other half serving to illuminate the speci¬ men S, in the glass cell. The distance between the condenser and the specimen should be from 50 to 100 cm. (2 to 4 feet). Either before or after pas¬ sing through the specimen S, (preferably before, as in fig. 375) this light strikes the special condenser C'j, which brings the diaphragm Di, to a focus at D2. At this point is placed the diaphragm Do, which is so arranged as to just cut off the remainder of the light, its edge coming to the edge of the image of the dia¬ phragm Di. The objective O, is focused to bring the specimen S, to a sharp focus on the screen before the diaphragm D2, is in place. With the apparatus thus arranged the screen will be perfectly dark, all light not intercepted by the first diaphragm being stopped by the second. If, now, the liquid in the cell S, is not quite homogeneous but is cordy, as when glycerine and water are first mixed or when a crystal of salt is dissolving, the image of Dj, will not Fio. 375. Dark Ground Illumination; Toepler Method of Striae. L Arc- C, First part of the magic lantern condenser. D, Diaphragm. C2 Condenser of long focus. It must be as perfect a lens as can be found. .S’ Specimen, with slight inhomogeneity. a An inhomogencity in the specimen which deviates the light. D2 Diaphragm intercepting direct light from the lantern. O Objective. a' Image of a. Note that with the objective on the axis only the upper por¬ tion of the objective is used.