. The microscope; an introduction to microscopic methods and to histology. Microscopes; 1899. CM. II.] LIGHTING AND FOCUSING. 47 § 88. Dark-Ground Illumination.—When an object is lighted with rays of a greater obliquity than can get into the front lens of the objec- tive, the field will appear dark (Fig. 51 ). If now the object is cotn-. 48. 49- 50. 51- Figs. 48-51. Sectional views of the Abbe Illuminator of N. A. showing various methods of illumination {\ 84). Fig. 4S, axial light with parallel rays. Fig. 49, oblique light. Fig. 50, axial light with converging beam. Fig. 51, dark- ground
. The microscope; an introduction to microscopic methods and to histology. Microscopes; 1899. CM. II.] LIGHTING AND FOCUSING. 47 § 88. Dark-Ground Illumination.—When an object is lighted with rays of a greater obliquity than can get into the front lens of the objec- tive, the field will appear dark (Fig. 51 ). If now the object is cotn-. 48. 49- 50. 51- Figs. 48-51. Sectional views of the Abbe Illuminator of N. A. showing various methods of illumination {\ 84). Fig. 4S, axial light with parallel rays. Fig. 49, oblique light. Fig. 50, axial light with converging beam. Fig. 51, dark- ground illumination with a central stop diaphragm. Axis. The optic axis of the illuminator and of the microscope. The illumina- tor is centered, that is its optic axis is a prolongation of the optic axis of the microscope. S. Axis. Secondary axis. In oblique light the central ray passes along a sec- ondary axis of the illuminator, and is therefore oblique to the principal axis. D D. Diaphragms. These are placed in sectional and in face views. The dia- phragm is placed between the mirror and the illuminator. In Fig. 49 the opening is eccentric for oblique light, and in Fig. 5/ the opening is a narrow ting, the central part being stopped out, and thus giving rise to dark-ground illumination (188). Obj. Obj. The front of the objective. posed of fine particles, or is semi-transparent, it will refract or reflect the light which meets it, in such a way that a part of the very oblique rays will pass into the objective, hence as light reaches the objective only from the object, all the surrounding field will be dark and the ob- ject will appear like a self-luminous one on a dark back-ground. This. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Gage, Simon Henry, 1851-1944. Ithaca, N. Y. , Comstock Publishing Company
Size: 1923px × 1299px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookauth, bookcentury1800, bookdecade1890, booksubjectmicroscopes
