The Cu)npou}id Microscope 89 



(engraved on them); 5X and lOX are the customary equipment for 

 student and laboratory microscopes — the former for low power over 

 large areas, the latter for general ^vork. Most of these microscopes are 

 equipped Avith three objectives: 50 to 32 mm. (low power); 16 mm. 

 (middle or intermediate power); and 8 to 4 mm. (high power). All are 

 engraved with their magnifying power. An additional objective Avill be 

 included on the four-objecti\e nosepiece and can be used interchange- 

 ably with the \o\v power (or any other poAver) on the three-objective 

 nosepiece. As one shifts from low power to higher po^vers, the magnifica- 

 tion increases but the size and depth of the exainining field decreases; 

 also, the clearance of the objective above the specimen (working dis- 

 tance) decreases, and more light is required. The working distance is 

 the distance between the front of the objective and the top of the cover 

 glass Avhen the specimen is in focus. 



Achromatic objectives (producing an image essentially free of color 

 fringes) are found on most laboratory microscopes. These objectives 

 are corrected chromatically for the light of two wavelengths, the C 

 (red) and F (blue) lines of the spectrum and spherically for the light of 

 one color, usually in the yellow greens. Achromats give a good image 

 in the central portion of the field but not as sharp an image at the edges 

 as can be obtained Avith apochromatic objectives. The latter are essential 

 for critical microscopy and color photography because they are cor- 

 rected for three colors, including the G (violet) lines of the spectrum, 

 thereby reducing the amount of color fringes. Lenses, if not corrected 

 in this manner, exhibit chromatic aberration, and color fringes will 

 confuse the border of the specimen image. For best results apochromatic 

 objectives should be accompanied by a corrected condenser of a nu- 

 merical aperture at least equal to that of the objective aperture and 

 compensating ocidar eyepieces. A correction collar on the apochromatic 

 objectives can be adjusted to compensate for the thickness of the cover 

 glass on the slide. The collar must be used and can be adjusted with one 

 hand while the fine adjustment is manipulated with the other hand. 



The body tube supporting the lenses may have a fixed length or it 

 may be equipped with an adjustable drawtube. A scale on the latter is 

 used to determine the length of the tube for the various lenses used in 

 it — usually 160 mm. for American-made objectives. This becomes more 

 critical with higher power objectives than with lower power ones for 

 improving sharpness of detail luider a thick cover glass. 



At the bottom of the microscope is located a mirror to direct light 

 through the diaphragm and into the condenser. Usually there are two 

 mirrored surfaces: one plane (flat) to direct the light, reflecting a 



