80 REFRACTION AND COLOR IMAGES [Ch. Ill 



with the microscope to find out for himself for what tube-length each 

 of his objectives was corrected (see Ch. IX). 



§ 136. Water immersion objectives. — Put a water immersion 

 objective in position (§ 44) and the fly's wing for object under the 

 microscope. Place a drop of distilled water on the cover-glass, and 

 with the coarse adjustment lower the tube till the objective dips into 

 the water, then light the field well and turn the fine adjustment one 

 way and another till the image is clear. Water immersions are ex- 

 ceedingly convenient in studying the circulation of the blood, and for 

 many other purposes where aqueous liquids are liable to get on the 

 cover-glass. If the objective is adjustable, follow the directions given 



in § 135- 



When one is through using a water immersion objective, remove 

 it from the microscope and with some lens paper wipe all the water 

 from the front lens. Unless this is done dust collects and sooner or 

 later the front lens will be clouded. It is better to use distilled water 

 to avoid the gritty substances that are liable to be present in natural 

 water, as these gritty particles might scratch the front lens. 



Refraction and Color Images 



§ 137. Refraction images are those mostly seen in studying micro- 

 scopic objects. — ■ They are the appearances produced by the refrac- 

 tion of the light on entering and on leaving an object. They therefore 

 depend (a) upon the form of the object, (b) upon the relative refrac- 

 tive powers of object and mounting medium. With such images the 

 diaphragm should not be too large (see § 106-107). 



If the color and refractive index of the object were exactly like the 

 mounting medium, it could not be seen. In most cases both refractive 

 index and color differ somewhat ; there is then a combination of color 

 and refraction images which is a great advantage. This combina- 

 tion is generally taken advantage of in histology. The air bubble in 

 § 194 is an example of a purely refractive image. 



A purely refractive image like that given by an air bubble or a fat 

 globule gives a dark border for central transmitted light, and a light 

 border on a black field with very oblique light, such as is given by the 

 mirror turned far to one side or by a central stop when the condenser 



