Ch. I] FUNCTION OF AN OBJECTIVE 3 1 



Function of an Objective 



§ 50. Put a 50 mm. objective on the microscope, or screw off the 

 front combination of a 16 mm., and put the back combination on the 

 microscope for a low objective. 



Place some printed letters or figures under the microscope, and 

 light well. In place of an ocular put a screen of ground-glass, or a 

 piece of lens paper, over the upper end of the tube of the microscope. 



Lower the tube of the microscope by means of the coarse adjust- 

 ment until the objective is within 2 to 3 cm. of the object on the stage. 

 Look at the screen on the top of the tube, holding the head about as 

 far from it as for ordinary reading, and slowly elevate the tube by 

 means of the coarse adjustment until the image of the letters appears 

 on the screen. 



The image can be more clearly seen if the object is in a strong light 

 and the screen in a moderate light, i.e., if the top of the microscope is 

 shaded. 



The letters will appear as if printed on the ground-glass or paper, 

 but will be inverted. 



If the objective is not raised sufficiently, and the head is held 

 too near the microscope, the objective will act as a simple microscope. 

 If the letters are erect, and appear to be down in the microscope 

 and not on the screen, hold the head farther from it, shade the screen, 

 and raise the tube of the microscope until the letters do appear on 

 the ground-glass. 



§ 50a. Ground-glass may be very easily prepared by placing some fine 

 emery or carborundum between two pieces of glass, wetting it with water, and 

 then rubbing the glasses together for a few minutes. If the glass becomes too 

 opaque, it may be rendered more translucent by rubbing some oil upon it. 



§ 51. Aerial image. — After seeing the real image on the ground- 

 glass or paper, use the lens paper over about half of the opening of 

 the tube of the microscope. Hold the eye about 250 mm. from the 

 microscope as before and shade the top of the tube by holding the hand 

 between it and the light, or in some other way. The real image can 

 be seen in part as if on the paper and in part in the air. Move the 

 paper so that the image of half a letter will be on the paper and half 



