3 MICROSCOPE AND ACCESSORIES. 



magnifier (Plate II, Fig. 9) is one of the best forms. For many pur- 

 poses a special mechanical mounting like that of Fig. 8 a is to be pre- 

 ferred. 



COMPOUND MICROSCOPE. 



MECHANICAL PARTS. 



I 5. The Mechanical Parts of a laboratory, compound microscope are shown in 

 PI. II, Fig. 10, and are described in the explanation of that figure. The student 

 should study the figure with a microscope before him and become thoroughly fa- 

 miliar with the names of all the parts. 



OPTICAL PARTS. 



\ 6. Microscopic Objectives. — These consist of a converging lens or of one or 

 more converging lens-systems, which give an enlarged, inverted, real image of the 

 object (PI. I, Fig. 3 and 5). And as for the formation of real images generally, the 

 object must be placed outside the principal focus, instead of within it, as for the 

 simple microscope. (See \\ 4, 34). 



Modern microscopic objectives usually consist of two or more systems or com- 

 binations of lenses, the one next the object being called the front combination or 

 lens, the one farthest from the object and nearest the ocular, the back combina- 

 tion or system. There may be also one or more intermediate systems. Each 

 combination is, in general, composed of a convex and a concave lens. The com- 

 bined action of the systems serves to produce an image free from color and from 

 spherical distortion. In the ordinary achromatic objectives the convex lenses are 

 of crown and the concave lenses of flint glass (Pi. II, Fig. 12, 13). 



NOMENCLATURE OR TERMINOLOGY OF OBJECTIVES. 



\ 7. Equivalent Focus. — In America, England, and sometimes also 011 the 

 Continent, objectives are designated by their equivalent focal length. This 

 length is given either in inches (usually contracted to in.) or in millimeters (mm.). 

 Thus : An objective designated y 1 ^ in. or 2 mm., indicates that the objective pro- 

 duces a real image of the same size as is produced by a simple converging lens 

 whose principal focal distance is T V inch or 2 millimeters (PI. I, Fig. 1). An 

 objective marked 3 in. or 75 mm., produces approximately the same sized real im- 

 age as a simple converging lens of 3 inches or 75 millimeters focal length. And 

 in accordance with the law that the relative size of object and image vary directly 

 as their distance from the center of the lens (PI. I, Fig. 2, 3), it follows that the 

 less the focal distance of the simple lens or of the equivalent focal distance of the 

 objective, the greater is the size of the real image. 



\ 8. Numbering or Lettering Objectives. — Instead of designating objectives by 

 their equivalent focus, many Continental opticians use letters or figures for this 

 purpose. With this method the smaller the number, or the earlier in the alpha- 

 bet the letter, the lower is the power of the objective. (See further in Ch. Ill, 

 for the power or magnification of Objectives.) This method is entirely arbitrary 

 and does not, like the one above, give direct information concerning the ob- 

 j ective. 



