Bauhaus-Universität Weimar

Titel:
Optic Projection: Principles, Installation and Use of the Magic Lantern, Projection Microscope, Reflecting Lantern, Moving Picture Machine
Person:
Gage, Henry Simon and Henry Phelps Gage
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit39438/656/
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.
        

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