MANIPULATION OF THE MICROSCOPE. 31 
the ring widening and the center becoming sharper as 
the objective passes downward. With the oil-bubble 
this. is reversed, the brightest center and the widest, 
ring being above. Why this is so will be seen from 
Fig. 18. The oil-bubble, a dense sphere in a less 
dense medium, acts like an ordinary lens concentrating 
light to a bright focus above, the surrounding ring from 
which the light is diverted being dark. This dark ring 
will widen upward. The air-bubble, on the other hand, 
being less dense than the mucilage, diverges the rays of 
light, and its dark ring is widest below. 
4. The Use of the Draw-tube.—The spherical and 
chromatic aberration of an objective vary with its rela- 
tion to the object examined and to the eyepiece. There- 
fore when we say that in an achromatic objective these 
defects have been corrected, this is only true for a certain 
standard set of conditions. If we change the position of 
the draw-tube and therefore the distance between the 
objective and eyepiece systems, a certain amount of aber- 
ration is again introduced. For each objective there is a 
certain proper tube-length, record of which accompanies 
the microscope and may be placed upon the objective 
itself. According to the best American practice the 
tube-length, measured between the upper end of the tube 
where the eyepiece is inserted and the lower end of the 
tube where the objective is inserted, is either 160 or 
216 mm. 
A cover-glass is commonly placed between the object 
and the objective in order to hold the former in place 
and to protect the latter; and this introduces a pertur- 
