disturbing the focus more than the fine adjustment 

 will easily rectify. 



Eyepieces are divided into two classes : 



Negative eyepieces, Fig. 39, in which the focal point 

 is within the eyepiece itself and between eye lens e I 

 and field lens //or at the diaphragm d d. 



Positive eyepieces, Fig. 40, in which the focal point 

 is outside and below the field lens f 1. These eye- 

 pieces can be used as magnifiers. 



Huyghenian Eyepiece. This form is named 

 after Huyghens, who is said to have first used it. It is 

 the construction which is in most general use, although 

 made up in a variety of mountings. It is negative and 

 consists, as has already been stated, Fig. 39, of an 

 eye lens, e I, nearest the eye, and a field or collective 

 lens, f I, which is the large lens nearest the objective. 

 Between them and placed at the focus of the eye lens is 

 a perforated, blackened disc, called a diaphragm, d d, 

 which limits the size of field and shows it within a 

 sharply defined border. The Huyghenian eyepiece is 

 made in two forms : 



The English type as shown in Fig. 41, which has 

 a large tube fitting into the microscope tube and a 

 neck or smaller tube which is usually arranged with 

 a cap to slide over the eye lens. 



The Continental type, Fig. 42, has a straight tube 

 which drops entirely into the tube of the microscope 

 and rests upon the mounting of the eye lens. 



107 



