Ch. Ill] REFRACTION AND COLOR IMAGES 81 



is used (§ 123, 201). In both cases the object is in outline. As 

 pointed out by Wright (p. 5,41) the visibility of the object shown in 

 outline depends on the width of the outline and not on the diameter 

 of the whole object. If the width of the outline is too narrow to in- 

 clude the necessary visual angle of 1 minute (§ 227) the whole object 

 fades into the background and is no longer visible. On the other 

 hand, if the object is colored, then it is visible so long as its entire 

 diameter gives a visual angle of 1 minute or more. 



One can see from the above what a tremendous advantage it 

 is in studying the finest details of structure to have them brilliantly 

 colored. 



Homogeneous Immersion Objectives 

 Experiments 



As stated above (§ 25), these are objectives (fig. 21B) in which a 

 liquid of the same refractive index as the front lens of the objective 

 is placed between the front lens and the cover-glass. 



§ 138. Refraction images. — Put a 2 mm. homogeneous immersion 

 objective in position; employ a condenser. Use some histological 

 specimen like a muscular fiber as object; make the diaphragm open- 

 ing about 9 mm. in diameter, add a drop of the homogeneous immer- 

 sion liquid, and focus as directed in § 72. The object will be clearly 

 seen in all its details by the unequal refraction of the light traversing 

 it. The difference in color between it and the surrounding medium 

 will also increase the sharpness of the outline. If an air bubble prepa- 

 ration (§ 195) were used, one would get pure refraction images. 



§ 139. Color images. — Use some stained bacteria as Bacillus 

 tuberculosis for object. Put a drop of the immersion liquid on the 

 cover-glass or on the front lens of the homogeneous objective. Re- 

 move the diaphragms from the illuminator or in case the iris diaphragm 

 is used, open it to its greatest extent. Focus the objective down so 

 that the immersion fluid is in contact with both the front lens and the 

 cover-glass; then with the fine adjustment get the bacteria in focus. 

 They will stand out as clearly defined colored objects on a bright 

 field. 



