Magnifying Power 5S 



the objective, and the eyepiece or ocular. Under the head 

 of ' Objectives ' in the makers' catalogues it will be noticed 

 that the powers are expressed as 1-inch, ^-inch, |-inch, 

 etc. The figures do not indicate the distance at which 

 the lenses focus on the object, but are intended to ap- 

 proximately convey the equivalent focus, and thereby the 

 actual magnifying power of the objective. To understand 

 this description, imagine an objective to be placed so as 

 to form an image of an object at 10 inches from its back 

 lens.* Then, an objective which, when so placed, formed 

 an image on the screen which was ten times (diameters) the 

 real size of the object would be described as a 1-inch 

 objective ; one which formed an image twenty times the 

 size of the object would be called a J-inch objective, and in 

 general the result given, when the magnification of the 

 image formed on the screen is divided into ten, is what is 

 spoken of as the focal length of the objective; also the 

 equivalent focus of an objective divided into ten gives its 

 magnifying power — thus a 2-inch should magnify 5, a 

 ^-inch 40, and a ^-inch 80, diameters. This is termed the 

 initial magnifying power of an objective. 



The foci of German objectives are usually expressed in 

 millimetres, 250 millimetres (about 9f| inches) being taken 

 as the normal vision distance, and the focal length of the 

 objective divided into 250 gives the initial magnifying 

 power.* Thus a 3-millimetre objective should have an 

 initial power of 83|^, and a 4-millimetre of 62|, diameters, 

 and so on. 



The image formed by the objective is again magnified by 

 the eyepiece. Unfortunately, the latter is rarely marked 

 with its magnifying power, the general rule being to call 

 the different powers by the letters A, B, C, D, etc., or 1, 2, 



* The above plan will be sufficiently accurate for experimental purposes, 

 but, strictly speaking, it is the equivalent focus of the objective which 

 determines its magnifying power, and in order to obtain exact results the 

 measurements should be taken from the upper focal plane of the objective. 

 The optical tube-length should also be reckoned in like manner, and this may 

 generally be assumed to be from J inch to 1 inch longer than the mechanical 

 tube-length or the length of the body of the microscope, 



