OF THE MICROSCOPE. 3 



ordinary French objectives is about a quarter of an inch in 

 focal length. As it is often useful to the microscopist to know 

 the powers of the different objectives made by prominent con- 

 tinental makers, we give the focal lengths of the objectives of 

 Nachet, Hartnack, and Gundlach, premising, however, that 

 by so doing we by no means intend to class these objectives 

 with ordinary French triplets. 



Nachet's ordinary objectives are as follows: 



Number 12345 



Focus in inches 2 1 1-2- 1-4 1-5 1-8 



The immersion and correction objectives of the same maker 

 are as follows: 



Number 6 7 8 9 10 11 12 



Focus in inches 1-10 1-14 1-15 1-26 1-30 1-40 1-50 



Hartnack's objectives of recent construction are as follows: 



Number 123456789 



Focus in inches.... 2 1 3-4 1-2 1-4 1-5 1-6 1-9 1-11 



Hartnack's new objectives with immersion and correction are 

 as follows: 



No. Focus in inches.* 



9 1-12 



10 , 1-16 



11 1-18 



12 1-21 



13 .. 1-25 



No. Focus in inches. 



14 1-28 



15 1-33 



16 1-40 



17. 1-45 



18.. . 1-50 



The following is Gundlach's scale: 



No. Focus in inches. ( No. Focus In inches. 



I.... , 1 Via . .. 1-12 



II 1-2 



III 1-3 



IV 1-4 



V .1-8 



Vlb 1-12 



Vila 1-16 



VII6 1-16 



Yin 1-24 



IX . . 1-32 



Via and Vila are not adjustable for thickness of cover, 

 while VI6 and VII are. Vila, VII6, VIII and IX are immer- 

 sion lenses. 



