THE MICROSCOPE. 
But much trouble will be produced when objects of the smallest 
class requiring the higher powers are examined with instru¬ 
ments in which the stage-screws and fine adjustments are not 
supplied. 
39. In all cases it will be found advantageous to submit the 
object successively to a series of increasing powers. When once 
centred it w r ill maintain its place w 7 hile the object-lenses are 
changed, so that upon each change of power no new adjustment is. 
necessary except focussing. The low powers will show the 
general form and contour, the entire object being at one and the 
same moment within the field. The next powers will show the 
larger parts, and the highest will display the texture of the sur¬ 
face and the structure of the smaller parts. By working the 
stage-screws the object is moved like a panorama across the field 
from right to left; and this motion is repeated for various posi¬ 
tions given to it by the screws, which move it backward and 
forward until every part of it has been submitted to examination. 
When high powers are used the object will be very close to 
the object-glass, so as almost to touch it when the instrument is 
focussed. In this case, care should be taken to prevent all 
contact or friction of the object or the slide with the object-glass, 
the latter being subject from that cause to injury or fracture. 
When it is desired therefore to change an object thus viewed with 
a high power, it is always advisable to separate the object-glass 
and stage by the coarse adjustment, before removing the one 
object and replacing it with the other, which must then be 
focussed. 
40. The greatest care should be taken to clean the slides before 
placing them on the stage, since the least particle of grease or 
dust or any other foreign matter would, when magnified, injure 
the observation and might lead to errors. 
When the object observed is in a drop of water or other liquid, 
or when it is itself a liquid, it will be included between the 
slide and a thin glass placed upon it, in which case it is of the 
greatest importance to exclude or remove all bubbles of air, since 
they would present appearances under the microscope, such as 
would deface those of the proper object of observation. 
41. When it is required to submit a minute object to inspec¬ 
tion, it is sometimes desirable to submit it to pressure, either to 
retain it in one position, if it be living, or to ascertain the effect 
of compression upon it, exercised in a greater or less degree for 
other purposes. It is often necessary also to roll it over, so as to 
present all sides of it in succession to the observer. 
An instrument called a compressor has been contrived for this 
purpose, which has been constructed in a great variety of forms 
40 
