CH. /.] MICROSCOPE AND ACCESSORIES. 29 



2 m nt 



• 



Fig. 37. Figures showing approximately the actual size of the field with ob- 

 jectives 0/85 mm., /5 Dim., 17 mm., 5 mm., and 2 mm., equivalent focus, and 

 ocular of 37 '2 mm., equivalent focus in each case. This figure shows graphically 

 what is also very clearly indicated in the table (? //). 



§ 48. The size of the field of the microscope as projected into the 

 field of vision of the normal human eye ( 7. e., the virtual image) may 

 be determined by the use of the camera lucida with the drawing surface 

 placed at the standard distance of 250 millimeters (Ch. IV). 



FUNCTION OF AN OBJECTIVE. 



§ 49. Put a 2-in. (50 mm. ) objective on the microscope or screw off 

 the front combination of a 16 mm., ( ; j-in. ), and put the back-combina- 

 tion 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 adjustment 

 until the objective is within 2-3 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, 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 (Fig. 21). 



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. 



* Ground glass may be very easily prepared by placing some fine emery 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. 



