Appendix A : The Microscope and Its Optical Principles 157 



numerical aperture). Resolving power is tested by the resolution of fine 

 parallel lines ruled on glass or the striae on the surface of diatoms. The 

 test is to determine how many lines to the inch or centimeter may be 

 distinguished, and whether the objective simply glimpses the markings 

 or whether it resolves them clearly. The wider the angle of aperture, the 

 better the resolving power, provided the width is not so great as to inter- 

 fere with the correction of the lenses. The increased resolution of immer- 

 sion lenses is due to the fact that the immersion fluid practically widens 

 the angle of aperture (see immersion objective). 



Tube-Length. The distance between the places of insertion of ocular 

 and objective into the tube of the microscope. There are two standard 

 tube-lengths; the short standard is 160 mm. (6-1% inches), the long 

 standard, 216 mm. (8-^0 inches). Many makers, however, do not adhere 

 to the standards. The optical efficiency of the instrument is the same 

 in either case. The short length is more advantageous in that it is more 

 compact. The lenses must be corrected for the length of tube with 

 which they are to be used. The short standard is in use in most 

 American laboratories. 



Overcorrection and Undercorrection. In correcting for chromatic aberra- 

 tion, if the concave lens is stronger than is necessary to neutralize the 

 aberration of the convex lens, the blue rays are brought to focus beyond 

 the true principal focus of the objective, and the latter is said to be over- 

 corrected; if the concave lens is not strong enough, the result is what is 

 known as undercorrection. In case of overcorrection, the object takes on 

 an orange tint if, after focusing, the distance between object and 

 objective is slightly increased ; or it becomes of bluish color if the distance 

 is decreased. In case of undercorrection just the reverse is true. In some 

 instances the objective is purposely undercorrected, and the eyepiece 

 (e. g., compensating ocular) is equally overcorrected. 



Working-Distance. The distance between the front lens of the objective 

 and the object when the latter is in focus. With high powers it is very 

 small, so that with some oil-immersion objectives if a thick cover is used 

 it is impossible to focus upon the object. For this reason thin cover- 

 glasses (No. 1) should be used on preparations which are to be used with 

 high-power immersion lenses. 



MANIPULATION OF THE COMPOUND MICROSCOPE 



1. Always handle the instrument cautiously; it is a delicate 

 mechanism. Lift it by the base, not by the tube or the arm. 



2. The work-table should be of such a height that the observer 

 can sit at it comfortably without compressing the chest or tiring 



