114 



They are divided into two classes : 



Negative eyepieces in which the focus is within 

 the eyepiece itself (at the diaphragm). 



Positive eyepieces, in which the focus is outside 

 and below the field lens and which can be used as 

 magnifiers. 



Huyghenian Eyepieces. This is named after 

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

 a construction which is in most general use, 

 although made up in a variety of mountings. It 

 is negative and consists, as we have already stated, 

 of an eye lens, nearest the eye and field lens or col- 

 lective which is the large lens nearest the objec- 



Fig. 36. 



tive. Between them and placed as the focus of 

 the eye lens is a perforated, blackened disk, called 

 a diaphragm, which limits the size of field and 



