80 



four and five. Two are generally used in low 

 powers, three and sometimes four in the medium 

 powers and four to five in the high powers. They 

 are the individual portions consisting of one, two 

 or three lenses, which, when composed of more 

 than one lens, are cemented together and make up 

 the objective. An achromatic single system objec- 

 tive may consist of two or three lenses and a three 



M 

 A 



Fig. 29. 



or four system may consist of as many as eight or 

 ten lenses. The systems are called in their order : 

 anterior or front, middle &&& posterior . When one 

 consists of two lenses it is called a doublet, when 

 of three lenses a triplet. Thus in Fig. 29. A is 

 the anterior, M the middle and P the posterior 

 system ; thus also A is a single system, M a 

 double and P a triple one. 



