The microscope : its history, construction, and application, being a familiar introduction to the use of the instrument, and the study of microscopical science / by Jabez Hogg.

  • Hogg, Jabez, 1817-1899.
Date:
1898
Catalogue details

Licence: Public Domain Mark

Credit: The microscope : its history, construction, and application, being a familiar introduction to the use of the instrument, and the study of microscopical science / by Jabez Hogg. Source: Wellcome Collection.

Provider: This material has been provided by University of Bristol Library. The original may be consulted at University of Bristol Library.

    52/780 (page 20)
    In the case of a concave lens (Fig. 10), rays incident parallel to the principal axis diverge after passing through; and their directions, if produced backwards, wovild approximately meet in a point F; this is its principal focus. It is, however, only a virtual focus, inasmuch Fig. 10.—rriuciiml Focus of Concave Lens. as the emergent rays do not actiially pass through it, whereas the principal focus of a converging lens is real. Optical Centre of a Lens.—Secondary Axes.—Let 0 and 0' (Fig. 11) be the centres of the two spherical surfaces of a lens. Draw any two parallel radii, 0 I, 0' E, to meet these surfaces, and let the joining line I E represent a ray passing through the lens. This ray makes equal angles with the normals at I and E, since these latter are parallel by constinc- tion; hence the incident and emergent rays S I, E R also make equal angles with the normals, and are therefore parallel. In fact, if tangent planes (indicated by the dotted lines in the figure) are drawn at I and E, the whole coui-se of the ray S I E 11 will be the same as if it had passed through a plate bounded by these planes. Let C be the point in which the line I E cuts the principal axis, and let R, R' denote the radii of the two spherical surfaces. Then from the similarity of the triangles 0 C I, 0' C E, we have 0 ^ f o' . \ K Fig. 11. -Principal Centre of Lens.