21 MICROSCOPE AND ACCESSORIES. 



objective, have the illuminator on the level or nearly on the level of the 

 upper surface of the stage, and use the plane mirror. Be sure that the 

 diaphragm carrier is in the notch indicating that it is central in position. 

 Use the Pleurasigma as object. Study carefully the appearance of the 

 diatom with this central light, then make the diaphragm excentric so 

 as to light with oblique light. The differences in appearance will prob- 

 ably be even more striking than with the mirror alone (§§ 52, 53). 



§ 59. Lateral Swaying of the Image. — Frequently in studying an 

 object, especially with a high power, it will appear to sway from side 

 to side in focusing up or down. A glass stage micrometer or fly's wing 

 is an excellent object. Make the light central or axial and focus up 

 and down and notice that the lines simply disappear or grow dim. Now 

 make the light oblique, either by making the diaphragm opening excen- 

 tric or if simply a mirror is used, by swinging the mirror sidewise. On 

 focusing up and down, the lines will sway from side to side. What is 

 the direction of apparent movement in focusing down with reference to 

 the illuminating ray ? What in focusing up ? If one understands this 

 experiment it may sometime save a great deal of confusion. 



§ 60. Dark-Ground Illumination. — When an object is lighted with 

 rays of a greater obliquity than can get into the front-lens of the objec- 

 tive, the field will appear dark (PI. Ill, Fig. 22). If now the ob- 

 ject is composed of fine particles, or is semi-transparent, it will refract 

 or reflect the light which meets it, in such a way that a part of the very 

 oblique rays will pass into the objective, hence as light reaches the ob- 

 jective only from the object, all the surrounding field will be dark and 

 the object will appear like a self-luminous one on a dark back ground. 

 This form of illumination is only successful with low powers and objec- 

 tives of small aperture. It is well to make the illuminator immersion 

 for this experiment, see § 55. 



(A) With the Mirror. — Remove all the diaphragms so that very 

 oblique light may be used, employ a stage micrometer in which the 

 lines have been filled with graphite, use a ^ths (18 mm.) or No. 4 

 objective, and when the light is sufficiently oblique the lines will ap- 

 pear something like streaks of silver on a black back-ground. A 

 specimen like that described below in (B) may also be used. 



(B) With the Abbe Illuminator. — Have the illuminator so that the 

 light would be focused on the object (see above § 55) and use a dia- 

 phragm with the slit opening ; employ the same objective as in (A). 

 For object place a drop of a 10% solution of salicylic acid in alcohol 

 on the middle of a slide and allow it to dry and crystallize. The crys- 

 tals will appear brilliantly lighted on a dark back -ground. Put in an 

 ordinary diaphragm and make the light oblique by making the dia- 

 phragm eccentric. The same specimen may also be tried with a 



