46 LIGHTING AND FOCUSING [ CH. II 



Light well, and employ the proper opening in the diaphragm, 

 etc. (§ 77.) Look between the front of the objective and the" ob- 

 ject as before"(§ 81), and lower the tube with the coarse adjustment 

 till the objective almost touches the cover-glass over the object. 

 Look into the microscope, and with the coarse adjustment, raise the 

 tube very slowly until the image begins to appear, then turn the 

 milled head of the fine adjustment (frontispiece), first one way and 

 then the other, if necessary, until the image is sharply defined. 



In practice.it is found of great advantage to move the prepara- 

 tion slightly while focusing. This enables one to determine the 

 approach to the focal point either from the shadow or the color, if 

 the object is colored. With high powers and scattered objects there 

 might be no object in the small field (see § 57 Fig. 42 for size of 

 field). By moving the preparation an object will be moved across 

 the field and its shadow gives one the hint that the objective is ap- 

 proaching the focal point. It is sometimes desirable to focus on the 

 edge of the cement ring or on the little ring made by the marker 

 (see Figs. 70-75.) 



Note that this high objective must be brought nearer the ob- 

 ject than the low one, and that by changing to a higher ocular (if 

 the oculars are not par-focal) or lengthening the tube of the micro- 

 scope it will be found necessary to bring the objective still nearer 

 the object, as with the low objective. (For reason see Fig. 65.) 



§ 86. Always Focus Up, as directed above. If one lowers 

 the tube only when looking at the end of the objective as directed 



\ 85. Par-Focal Objectives. — By this is meant that the objectives are so 

 mounted that when changed on the microscope the object will remain approx- 

 imately in focus for all if it is in focus for any one. The expression is appli- 

 cable especially to a group of objectives on a revolving nose-piece. The 

 tube-length of the microscope must remain constant, for only a slight change 

 in length (10 to 15 mm.) will destroy the parfocalization. In case the 

 objectives on a revolving nose-piece are somewhat out of parfocalizatian one 

 may correct it by getting one in exact focus, and then noting when the others 

 are rotated in place whether the microscope must be focused up or down to 

 bring the objective in focus. 



If one winds a piece of string around the objective that is up too high it 

 will prevent it entering the nut of the nose-piece so far and hold it down at the 

 right level. 



It is not known by the writer who first thought of arranging the objectives 

 so that the different powers would be in focus when in position. It is a recent 

 improvement, coming in as a necessary consequence of parfocalizing the oculars. 



