THE MICROSCOPE. 21 



15. Lenses. These are so important that a detailed descrip- 

 tion of them is necessary. 



The Lenses and Lens-systems of the Microscope. 



Lenses. The lenses of an ordinary microscope are of 

 two kinds : those attached to the end of the tube nearer the 

 object, and known by the name of the objective lens, or 

 objective system of lenses, and. those fitting the end of the 

 tube into which the observer looks, known as the eye-piece 

 or ocular lens. 



The Objective Lens or System of Lenses. 



The objective is the principal lens or system of lenses of 

 the microscope. It is that which gives the greatest part of 

 the magnifying power to the instrument. As ordinarily 

 arranged, it is composed of a number of lenses connected 

 together in various ways, and known as combinations or 

 systems. The combination nearest the object is called the 

 front combination, or front lens, and that nearest the ocular 

 the back combination, or back lens. There may be one or 

 more intermediate systems between these. Each combination, 

 or system, consists of a concave lens of flint glass and a con- 

 vex lens of cro\vn glass ; the whole combination acts as a 

 double convex lens. The purposes of having lenses of vari- 

 ous shapes and materials is to correct what is known as 

 chromatic (colored) and spherical aberration or distortion (see 

 Fig. 1). 



Designation of the Objective. Objectives are designated, as 

 a rule, by their equivalent focal lengths. This length is usu- 

 ally given in inches or fractions thereof for instance, 1 inch, 

 J inch, ^ inch. In continental Europe the numerator of the 

 fraction is often omitted, the ^ objective being called 3, and 

 the | inch being called 7. These numbers indicate that the 

 objective produces a real image of the same size as is pro- 

 duced by a simple convex lens whose principal focal distance 

 would be that indicated by the number. And as " the rela- 

 tive size of object and image vary directly as their distance 



