84 THE AMERICAN MONTHLY [April, 



axial and focus up and down and notice that the lines simply dis- 

 appear or grow dim. Now make the light oblique either by mak- 

 ing the diaphragm opening eccentric or if simply a mirror is used 

 by swinging the mirror sidewise. On focussing up and down, the 

 lines will sway from side to side. What is the direction of ap- 

 parent movement in focussing down with reference to the illu- 

 minating ray ? What in focussing up ? If one understands this ex- 

 periment it may some time save a great deal of confusion. 



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

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

 objective, the field will appear dark (fig. 33). 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 tlie objective only from the object, all the surround- 

 ing field will be dark, and the object will appear like a self-lumi- 

 nous one on a dark back-ground. This form of illumination is only 

 successful with low powers and objectives of small aperture. It 

 is well to make the iUuminator immersion for this experiment. 



With the Abbe Illuminator. — Have the illuminator so that 

 the light would be focussed on the object and use a diaphragm with 

 the slit opening ; employ the three-fourths objective. For object 

 place a drop of a 10 per cent, solution of salicylic acid in alcohol 

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

 crystals will appear brilliantly lighted on a dark back-ground. 

 Put in an ordinary diaphragm and make the light oblique by mak- 

 ing the diaphragm eccentric. The same specimen may also be 

 tried with a mirror and oblique light. In order to appreciate the 

 difierence between this dark-ground and ordinary transmitted 

 light illumination, use an ordinary diaphragm and observe the 

 crystals. 



A very striking and instructive experiment may be made by 

 adding a very small drop of the solution to the dried preparation, 

 putting it under the microscope very quickly, lighting for dark- 

 ground illumination, and then watching the crystallization. 



HOMOGENEOUS IMMERSION OBJECTIVES : EXPERIMENTS. 



These are objectives 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. 



Put a 3 mm. (jVth in.) homogeneous immersion objective in 

 position, employ an Abbe illuminator. 



Refraction Images. — Use some histological specimen like a 

 muscular fibre as object, make the diaphragm opening only 

 slightly larger than the front lens, add a drop of the homogeneous 

 immersion liquid. The object will be clearly seen in all its de- 

 tails by the unequal refraction of the light traversing it. The 

 difierence in color between it and the surrounding medium will 

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

 aration were used, one would get pure refraction images. 



