70 PHOTOMICROGRAPHY 



any moderate deviation from this thickness will not 

 have much effect. But when using high-power dry lenses 

 it is necessary in critical work either to be careful to use 

 cover-glasses of the specified thickness, or to make an 

 adjustment for unavoidable differences found in various 

 mounts. Correction collars are fitted to many dry 

 objectives of J" and upwards, and the adjustment is 

 made by a slight alteration of the distance between the 

 component lenses of the objective. Alteration of the 

 tube-length of the microscope may also be employed to 

 obtain a suitable correction. If the cover-glass is too 

 thick, decrease the tube length, and if too thin, increase 

 it by means of the draw-tube until the best definition 

 is obtained. For instance, when a certain 4 mm. 0-95 

 N.A. objective, corrected for cover-glasses 0-18 mm. 

 thick and a 160 mm. tube, was used with thin covers of 

 0-15 mm., it was necessary to lengthen the tube by 30 mm. 

 It is a very difficult matter to make this adjustment, but 

 it may be done by examining a fine line or point in the 

 object, and focussing above and below it ; if the image 

 goes out of focus in the same manner both ways the 

 adjustment is correct. 



Oil-immersion lenses are not fitted with correction 

 collars, as the cedar oil and cover-slip are practically 

 one optical system, and, within wide limits, the thick- 

 ness of cover does not influence the performance of the 

 lens. 



Tube Length. All objectives are computed to perform 

 best with a definite tube length, that is distance between 

 the shoulder of the objective and the top of the tube on 

 which the eyepiece rests, indicated in Pig. 22 as the 

 " Mechanical tube length." Unfortunately, all makers do 

 not select the same tube length for which to correct 

 their objectives, but most lenses are now marked with the 

 standard adopted. The old English tube was 10", but this 

 is seldom used now ; most makers have adopted a length 



