THE MICROSCOPE. 135 



renders the object invisible — it destroys the so-called struc- 

 tural image. An uncolored object is rendered visible, because 

 of inequalities in the reflection, or refraction of the light, as 

 it strikes the object. On the other hand, a stained object 

 is visible because of the color. The color image, unlike 

 the structural image, is practically unaffected by the light 

 admitted through an open condenser. 



The following rule, therefore, should govern the use of 

 the condenser: When examining unstained preparations, as a 

 hanging-drop, the diaphragm should he restricted. When exam- 

 ining stained preparations, the diaphragm should remain open. 

 In the former case, the diaphragm should be restricted to 

 such an extent, that on looking into the microscope with 

 the objective in, or near the focus, it gives a dim twilight. 

 The diaphragm is contracted so that, at most, a pin-head 

 opening exists. When the specimen is stained, the dia- 

 phragm remains open. It is not advisable to open the dia- 

 phragm to the widest possible limit, since a stained object 

 is rendered more distinct if the diaphragm is slightly con- 

 stricted. 



In order to secure the best illumination of an object, it 

 is necessary (1) to set the plane mirror at the best angle for 

 reflection of the light, as it comes from a white cloud; (2) to 

 adjust the iris diaphragm, according, as the object is stained, 

 or colorless; (3) to focus the condenser on the object. Di- 

 rect sunlight should never strike the mirror, or the stage of 

 the microscope. 



The stand. — The various parts of the stand will be un- 

 derstood best by reference to the frontispiece. The draiv- 

 tube slides up and down in the body of the microscope. It 

 is graduated, and, when the instrument is about to be 

 used, the draw-tube should be raised to the line marked 

 17, provided no nose-piece is present. The upper face of 

 the eye-piece is then 170 mm. distant from the shoulder of 

 .the objective. This position should always be maintained 



