12 



THE MICROSCOPE 



The object 

 glass. 



The object glass (R) in the earliest instruments was a single 

 double convex lens (Fig. 2) ; it gave an enlarged but very im- 

 perfect picture of small objects, the outlines were ,j^^^^, 

 surrounded by coloured fringes, and the details "^^J-s:^ 

 were fuzzy and indistinct. Such lenses were made Fia. 2. 

 several himdred years ago, but in the early part 

 of the nineteenth century is was discovered that the defects of 

 a single lens could be overcome by using several lenses m 

 combination, made of difierent kinds of glass and of suitable 

 shapes and sizes. 

 Fooai length Modem object glasses are made of difierent powers to give 

 gtaB^°°* difierent magnifications in the primary image, and, in general, 

 the more an object glass magnifies, the larger the number of lenses 

 that are required to produce a perfect image. For instance. 

 Fig. 3 shows the optical construction of the 2/3-inch (16-mm.), 

 ^ ^ l/6-inch(4-mm.), 



and 1/12 - inch 

 (2-mm.) object 

 glasses. 



The name 



2/3, 1/6, or 1/12 



inch, as applied 



to an object 



glass, represents 



its focal length. 



It indicates its 



ma gnif ying 



power. If an 



ordinary single 



lens of 2-inch 



focal length is 



used as a hand magnifying glass, it has to be placed about 



2 inches from an object to give a clear image, and the 



2/3, 1/6, and 1/12 inch require to be placed at about these 



respective distances from the object when in use — thus the 



higher the magnifying power of a lens, the closer it must be 



to the object. 



Object glasses are not single lenses, but are composed of 

 several, and consequently the focal distance is measured from a 

 point about half-way between the front and back surfaces of the 

 component lenses. The distance between the foremost lens 

 and the object is, therefore, always considerably less than the 

 true focal distance. This is called the working distance to signify 

 the space between the end of the microscope and the object 

 when it is so adjusted that a clear picture is obtained, or when 

 it is, as it is caEed, " in focus." A list of the working distances 

 of difierent object glasses is given on page 82. 



An examination of the ^agram (Fig. 1) on page 9 illustrates 



Warldng 

 distance. 



Fio. 3. — •/ = focal length ; w = working distance. 



