14 THE MICROSCOPE 



duced by a lower power object glass which magnifies the object 

 ten times, and a higher power eyepiece which magnifies it 

 again by ten. The same result is obtained as far as magnifying 

 power is concerned, but a different result as regards the quality 

 of the image. 



Another method of varying the magnifying power is by increas- 

 ing the distance between the object glass and the eyepiece. To 

 enable this to be done the microscope is supplied with a sliding 

 drawtube (N), which allows the tube length to be varied from 

 140 to 200 mm. The reason for this increase in magnification 

 is well illustrated by reference to the lantern, in which case the 

 lantern lens gives a larger picture when it projects it upon a 

 screen that is at a greater distance. In the same way the micro- 

 scope object glass produces a more highly magnified primary 

 image if by sUght adjustment in the focussing of the instrument 

 the picture is formed at a greater distance, and the drawtube 

 of the microscope is extended so as to examine the picture 

 formed at this greater distance. 

 Field of The " field of view " is a term applied to the size of the object 



^^^' that can be seen at one time by means of the microscope. To 



assist in increasing the size of field an eyepiece is made of two 

 lenses instead of a single one. The lower field lens is situated 

 below the position U (Fig. 1), where the primary image is 

 produced, and increases the field of view while the upper lens 

 does the magnifying. 



Suppose that the apparent field of view is a circle of about 

 8 inches diameter at the position V, where the final image 

 seen through the microscope appears to be. It is evident that 

 with a microscope magnifying 100 diameters, the size of the largest 

 object that can be observed at one time is only 1/100 the size 

 of this field, or about 1/12 inch, so that for this reason alone 

 it is important that a microscope should possess a means of 

 varying the magnifying power. It is sometimes desirable to 

 examine a large area of an object with a small magnifying power, 

 at others a small area with a large magnifying power. A table 

 of the fields of view given by different lenses appears on page 82. 

 The question arises as to whether it is preferable to vary this 

 magnifying power by means of changing the eyepiece, by means 

 of changing the object glass, or by means of lengthening the 

 drawtube. 



This is influenced by an optical consideration of great 

 importance. 

 Aperture. In the early days, before it was understood how to correct 



the errors of a single lens, microscopes were constructed in 

 which the object glass was a single lens, the defects of which 

 were reduced by putting a very small aperture — almost a pin- 

 hole — in front or behind this lens. This meant that only an 

 extremely fine cone of light from each point of the object could 



