/Microscope Construction, Use, and Care 189 



style objecti\es should be left to the manul'acturers or to a skilled 

 insiiunient mechanic. The new Baiisch and Lomb objectives have 

 an internal adjustment, with which the student cannot tamper but 

 which can be easily adjusted with a special wrench. 



Dry objectives between 10 X and 60 X tan be made parlocal 

 in any combination. The older 4 to 5X objectives cannot be so 

 adjusted, but the American Optical Company now makes a 3.5 and 

 a 5.1 X objective, and Bausch and Lomb makes a 3.2 and a 6X 

 objective which can be made parlocal with the lOX. 'I'l^l parfocal 

 with the 43 X within a quarter turn of the fine adjustment. With a 

 combination of 3.2, 10, and 43 X objectives students should be taught 

 to change magnification up or down in that sequence, thereby mini- 

 mizing damage to slides and lenses. 



Types of Objectives 



ACHROMATIC OBJECTIVES 



These are in the lo-^vest price class and are used on classroom 

 microscopes and for routine work in research. In these oJDJectives 

 chromatic aberration is corrected lor two colors and s})hcrical aber- 

 ration for one color. Achromatic objectives have undergone relatively 

 greater improvement in recent years than have other types. 



FLUORITE OBJECTIVES 



In these objectives the mineral fluorite is used in conjunction with 

 special optical glasses. The corrections are of a higher order than 

 those of the achromatic series. Fluorite objectives are particularly 

 useful for photomicrography by virtue of excellent color correction. 

 Thev are aAailable only in magnifications over 40 X- 



APOCHROMATIC OBJECTIVES 



These objectives ha\e chromatic aberration corrected for three 

 colors and spherical correction for two colors, affording brilliant 

 images, presented in their true colors and without distortion of shape. 

 The highly actinic violet rays are brought to the same focus as the 

 longer visual rays, making these objectives highly desirable for pho- 

 tography. Apochromatic objectives are expensive because of their 

 complex construction and the scarcity of suitable fluorite. 



Oculars (Eyepieces) 



Oculars have distinctive optical characteristics that must be un- 

 derstood in order to use the correct ocular, and the correct combina- 

 tion of ocular and objective under specific conditions. An ocular has a 



