34 MICROSCOPE AND ACCESSORIES [ CH . I 



FUNCTION OF AN OBJECTIVE 



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

 the front combination of a 16 mm., (^i-in), and put the back com- 

 bination on the microscope for a low objective. 



Place some printed letters or figures under the microscope, and 



8 j m i 



FIG. 42. Figures shozuing approximately the actual size of the field with 

 objectives 0/85 mm., 45 mm., 77 mm., 5 mm. and 2 mm., equivalent focus, 

 and an ocular of 37 y z mm. equivalent focus in each case. This figure shores 

 graphically what is also very clearly indicated in the table (_\ 59}. 



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 micro- 

 scope* 



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

 justment 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 letter 

 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, z. e. , if the top of the micro- 

 scope is shaded. 



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

 but will be inverted (Fig. 26). 



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

 too near the microscope, the objective will act as a simple micro- 

 scope. If the letters are erect, and appear to be down in the micro- 

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



*$ 61. 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. 



