USE AND MANIPULATION OF MICKOSCOPE 155 



effect is, however, that a cover-glass introduces an aberration 

 which the objective has to correct. If the cover-glass is thicker 

 or thinner than the particular thickness for which the lens is 

 corrected, it would therefore influence the performance of the 

 lens ; the latter would not then give as good an image as when 

 working under proper conditions. In the case of apochromatic 

 lenses, particularly the 4-mm., and usually in the best quality 

 achromatic lenses of about one-sixth inch, provision is made, 

 by means of what is called a correction-collar, for altering the 

 distance between some of the component lenses of the objective 

 to compensate for variation in cover-glass thickness. Lenses 



FIG. 55. Influence of Cover-glass Thickness on Adjustment. 



are now usually corrected for a thickness of cover-glass of 0*17 

 to 0-18 mm., and any variation from this thickness introduces 

 an error which is comparable to spherical aberration. To 

 obtain the best results with any lenses, a cover-glass of the 

 exact thickness for which the lens is made, and which the makers 

 usually indicate in their catalogue or on the lens mount, should 

 be used. This, however, is rarely possible, so that adjustment 

 has to be made by the correction- collar provided on the objective 

 itself. An alternative method when such a correction-collar 

 is not provided, is to alter the tube-length that is, the distance 

 between the objective and the ocular. 



Oil-immersion objectives do not require any adjustment 



