OF THE MICROSCOPE. 57 



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 1 2 3 4 5 



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 1-20 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 1-4 1-6 1-9 1-11 



Hartnack's new objectives with immersion and correction are 

 as follows: 



Via and Vila are not adjustable for thickness of cover, 

 while VI6 and VII6 are. Vila, VH6, VHI and IX are immer- 

 sion lenses. 



Since taking up his residence in this country, Mr. Gundlach 

 has adopted the system of the English and American makers, 

 and designates his objectives by their focal length. The 

 table given above, however, will prove of service to those who 

 either possess or intend to purchase specimens of his earlier 

 work, some of which was very excellent. 



