4 ELEMENTARY CHEMICAL MICROSCOPY 



making up the object may be raised or lowered in the mount- 

 ing, thus affording a correction for the displacement of the image 

 brought about by the cover-glass. By consulting the diagram, 

 Fig. 1 8, page 40, it will be seen that by turning the collar C the 

 combination of lenses L will be displaced and their distance 

 from the combination L' will either be increased or diminished. 

 A spiral spring S holds the movable parts firmly in place. A 

 cover-glass which is thicker than that for which the objective 

 is corrected affects the image in the same manner as if the spheri- 

 cal aberration were over-corrected, while on the other hand if 

 too thin the effect produced is similar to that of under-correc- 

 tion. In the first case the focal distance of the objective must 

 be increased, and in the second, decreased. This is accomplished 

 by turning the adjusting collar to the right or left, as the case 

 may require, or, in the absence of such a device, by shortening 

 or lengthening the distance between the eyepiece and the objec- 

 tive, shortening for cover-glasses too thick, and lengthening for 

 those which are too thin. Fitting into the body tube of modern 

 microscopes is a tube which may be drawn out several centi- 

 meters. This tube is known as the draw-tube and is graduated 

 in millimeters. Objectives are commonly corrected (for use on 

 the usual type of microscope) for a tube length of 160 milli- 

 meters. 1 The 160-millimeter mark will therefore be found only 

 when the draw-tube is pulled out a short distance. This position 

 of the standard mark permits lengthening or shortening the 

 draw-tube, and thus correcting for cover-glass thickness as stated 

 above. 



In addition to corrections for chromatic and spherical aberra- 

 tion at least two other factors must be taken into account in 

 comparing, or choosing between, objectives of similar equivalent 

 focal length. These are the angular aperture and the numerical 

 aperture of the objectives. By the angular aperture of an objec- 

 tive is meant the " angle contained, in each case, between the 

 most diverging rays issuing from the axial point of an object 

 (i.e., a point in the object situated on the optic axis of the micro- 



1 Most metallographic microscopes, however, require objectives corrected for 

 200 mm. tubes and are designed to be employed without cover-glasses. 



