29 



the front of objectives and cover glasses properly 

 cleaned, in which respect water has the decided 

 advantage. 



It might be stated that such high power as ^th 

 and 2 V tn are vei 7 rare ly constructed at the present day, 

 and the^th may be considered the maximum, while 

 the T Vth is that most ordinarily used. This power will 

 give all the optical advantages, while higher powers 

 involve so many mechanical difficulties as to increase 

 the cost of production very considerably, and as a 

 rule detract from the optical qualities. 



A modern objective of the highest capacity may be 

 considered a work of art, and there are a few pro- 

 ductions of the human hand which exact so much 

 untiring application, ingenuity and skill. 



Systems. An objective is said to consist of sys- 

 tems which may vary in number from one to four 

 and five ; two and three are however mainly in use. 

 They are the individual portions consisting of one, 

 two or three lenses, which when more than one, are 

 cemented together and make up 

 the objective. An achromatic sin- 

 P gle system may consist of two or 

 three lenses, and a three or four 

 system objective may consist of 

 as many as seven or eight lenses. 

 The systems are called in their 

 A order : anterior or front, middle 

 and posterior. When one consists 

 of two lenses it is called a doublet, 

 when of three lenses a triplet. Thus in Fig. 8 ? A is 



