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 will maintain its place while 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 



