ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
115 
ing for the real image which is received upon a suitable surface, 
preferably that of a semi-transparent screen. 
( b ) Berger’s * * * § Method.— A variety of the preceding, in which an 
illuminated object and a collimating telescope at a finite distance are 
substituted for an indefinitely distant object, and the real image at the 
principal focus is sought by means of an observing Microscope. 
(c) Maskelyne’s f Method. — A telescope, fitted with crosswire eye- 
piece, is focused upon an object at indefinite distance. The (positive) 
lens under examination being then clamped over the object-glass, it is 
pointed to a near object, which is moved to such a position as will give 
an accurate image in the telescope, the position of the object being that 
of the principal focus of the lens. 
(d) Merz’s f Method. — A variety of the preceding, adapted to negative 
lenses. The telescope, with the lens under examination fitted in front 
of the object-glass, is focused upon an object at an indefinite distance, 
and then the lens being removed, is pointed to a near object which is 
moved to a position giving accurate focus. 
(e) Kerber’s § Method. — The positive lens being placed in front of 
an illuminated slit, it is moved to such a position that a slab of glass 
with parallel faces introduced into the path of the emerged rays produces, 
on being tilted, no change in the apparent position of the slit as viewed 
in a telescope ; this being a test of parallelism of the rays. 
None of the foregoing methods are applicable to very short focus 
lenses, and they give no information as to the Gauss points. 
(/) Pendlebury’s || Method. — The positions of each of the two 
principal foci are found objectively, and then the respective distances 
are found from these to two conjugate points by inserting a luminous 
object and finding, objectively, its image. Then calling these two 
distances p and q, the true focal length is found by Newton’s rule as a 
geometrical mean between them. After this calculation has been made, 
the distance between the two principal points can be found by subtract- 
ing twice the true focal length from the distance between the two 
principal foci. 
II. — Methods of Magnification. — (g) Ramsden’s Method. — Measure- 
ment is made of the size and distance of the real image formed at the 
conjugate focus of an object of known size, at known distance : and 
from these the focal depth is calculated. This method assumes the 
lens to be of negligible thickness. 
( h ) Meyerstein’s IT Method. — This is a modification of Ramsden’s 
* Berger, ‘Apparat zur genauer Bestimmimg der Brennweite von Objectiv- 
glaseru,’ Zeitsclir. f. Instrumentenk., vi. (1886) p. 272. 
t After considerable search I am unable to find any writing of Maskelyne’s 
which describes the method which goes by his name. The same remark applies to 
Ramsden’s method. 
X Merz, ‘ Ueber einen neuen Apparat zum Messen der Brennweite,’ Pogg. Ann , 
lxiv. (1845) p. 321. 
§ Kerber, ‘Verfahren zur Bestimmung der Brennweite von Linsen,’ Zeitsclir. 
f. Instrumentenk., i. (1881) p. 07. 
|| Pendlebury, ‘ Lenses and System of Lenses’ (London, 1884), p. 82. 
Meyerstein, ‘ Apparat zur Bestimmung der Brennweite spharischer Linsen und 
Liusensysteme,’ Wied. Annalen, i. (1877) p. 315 ; and Carl’s Repertorium, xiv. (1877) 
p. 303. 
