788 Botanical Microtechnique 



silicone immersion fluid is used to connect the immersion objective 

 with the slide, the values of n and sin u are both increased, and con- 

 secjuently the resolving power is increased. An N.A. of 1.10 can be 

 obtained with a water immersion lens, and N.A. 1.40 using cedar 

 oil. 



OPTICAL CORRECTIONS 



The foregoing discussion of the properties of objectives does not 

 take into account the quality of the image produced. A simple lens 

 produces a decidedly imperfect image. Rays of white light which 

 enter the lens are broken up to some extent into a band of colors, 

 a spectrum. These colors are not brought to a focus at a common 

 point; blue is converged at a point closer to the lens than is red. 

 Consequently, the colors of the object are not reproduced accurately, 

 and a color fringe or "rainbow" is \isible along the boundaries of 

 objects in the microscopic image. This is known as chromatic aber- 

 ration. 



Spherical aberration is a defect that produces poor definition in 

 the center of the field. This defect is aggravated by a cover glass 

 that is not within the maximum thickness range of 0.15-0.21 mm. 

 Image quality also is impaired by variation from the standard tube 

 length of 160 or 170 mm. designated by the manufacturer. Certain 

 objectives have an adjustable correction collar on the objective 

 mount, calibrated for variations in cover-glass thickness. 



Curvature ol ilie field is another structural defect in the micro- 

 scopic image. If the center of tlie field of \ iew is in sharp locus, the 

 margins may be out of focus. With some oi)jecti\es, the image may 

 be distinctly dome-sha})ed. Ihe degree to which object i\es are cor- 

 rected for the above color and structinal defects of the image will 

 be indicated in the discussion of the optical categories in which 

 objectives are classified. 



PARFOCALIZATION 



1 wo or more objectives are parfocal with eadi other when u is 

 [)ossible to locus one obiecti\e on an ()l)je(t, tmn lo the next objective 

 without focusing, and see the objed in more or less sharp focus. 

 This feature is extremely important wiili large classes of elementary 

 students. If the conventional lOX 1<>^\' power and the 10 lo l.")\ high 

 power are not parhxal. the siudtiii must rclociis with the latter 

 lens, \vhi(h has a short woiking distance, small luld. and shallow 

 depth ol focus. Excessive breakage of slides and scraidiing of objec- 

 tives o((ur if the ol)jectives are not parfocal. Adjusimcnt ol the old- 



