The microscope in theory and practice / translated from the German of Carl Naegeli and S. Schwendener.

Date:
1887
Catalogue details

Licence: Public Domain Mark

Credit: The microscope in theory and practice / translated from the German of Carl Naegeli and S. Schwendener. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

    33/402 (page 17)
    _ j\r* = X. Therefore, evidently, 77* = oc, and, as we assume that 7; > 0 has a finite value, Since, in this expression, the numerator is a finite quantity, the denominator must = 0, and, therefore, h — iV°) = nH, from which we e'et f = A'» + f • (16) In like manner for the analogous third case, in which the object- point lies at an infinite distance, we get from equation (13) the corresponding value of If, for instance, we consider that the quantities ti^ h and I, wliich occur in the numerator and denominator, disappear in comparison with the infinite quantity (^ — iV), it is readily seen that I* = i\r* _ (17) k These values of ^ and evidently correspond to the abscissfe of the two focal points i^and F^, and the planes drawn through them at right angles to the axis are the focal planes. As soon as one of the two points lies in the corresponding focal plane, the other moves to an infinite distance. The distances of the respective iDrincipal and focal points may be determined from their abscissae, given in equations (14) to (17), by simple subtraction of the corresponding values of the abscissae. We get E ~ F = - F» - F* = - (18) k The quantities - ■— and or the distances of the principal planes from the corresponding focal planes, are called the focal knfjtlis of the system. They have under all circumstances—since and n* are, from their nature, positive numbers—the same, and /.: the opposite, sign. If they are positive, and if, therefore, F lies behind F, and F^ behind F*, the system is called a convergent one ; it acts as a convergent lens and produces real images. In the C