12 MICROSCOPE AND ACCESSORIES. 



Some oculars, as the orthoseopic and periscopic, are so constructed as to 

 eliminate the ocular diaphragm, and in consequence, although this is 

 not the sole cause, the field is considerably increased. The exact size 

 of the field may be read off directly by putting a stage micrometer 

 under the microscope and noting the number of spaces required to meas- 

 ure the diameter of the light circle. 



The size of the field of the microscope as projected into the field of 

 vision of the normal human eye (z. e. , the virtual image) may be de- 

 termined by the use of the camera lucida with the drawing surface 

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



FUNCTION OF AN OBJECTIVE. 



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

 the front combination of a % -in. (18 mm.), 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 

 J apanese or tissue paper, over the upper end of the body of the micro- 

 scope.* 



I,o wer the body by means of the coarse adjustment (Fig. 10), until 

 the objective is within 2-3 cm. of the object on the stage. I,ook at the 

 screen on the top of the body, holding the head about as far from it as 

 for ordinary reading, and slowly elevate the body 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 (Fig. 5). 



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 latter, and raise the 

 body of the microscope until the letters do appear on the screen. 



To demonstrate that the object must be outside the principal focus 

 with the compound microscope, remove the screen and turn the tube of 

 the microscope directly toward the sun. Move the tube of the micro- 

 scope with the coarse adjustment until the burning or focal point is 



* 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 trans- 

 lucent by rubbing some oil upon it. 



