4 ELEMENTARY CHEMICAL MICROSCOPY 



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. The i6o-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 ob- 

 jective 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- 

 scope), that can enter the objective and take part in the formation 

 of an image" (Carpenter-Gage). 



This angle is obviously that of the cone of light rays whose 

 -apex lies in the optic axis of the microscope at the point where 

 the axis passes through the plane of the object and the diameter 

 of whose base is equivalent to the opening of the front lens com- 

 bination of the objective. 



Dry objectives may be compared with each other with refer- 

 ence to their angular aperture. In general the angular aperture 

 depends largely upon the diameter of the front combination of 

 the objective, and usually in objectives of like magnifying power, 

 the greater this diameter the larger will be the angular aperture 

 and the wider and clearer will be the area or field covered. It is 

 also generally true that the shorter the equivalent focus of the 

 objective, the larger its angular aperture and that dry objectives 

 of small working distance usually have large angular apertures. 

 It is obvious that in dry objectives an easy comparison of the 

 relative areas of field covered is afforded by a consideration of 

 .angular apertures. The true field of view of a compound mi- 



