54 



PRACTICAL MICROSCOPY. 



It may be useful also to add a list of the magnifying 

 power of the oculars and objectives of Hartnack (now 

 Prazmowski) and of Zeiss. They are used in this country 

 to some extent, and the users always have the foible 

 of omitting the number of diameters to which the object 

 has been magnified, adding, however, the numbers of both 

 ocular and objective, to which the student generally has no 

 reference. It should not be forgotten that the amplifying 

 powers of the foreign objectives are calculated for a tube 

 of 153 millimetres (6 inches) in length, and most of these 

 objectives lose in their performance if employed in the 

 ordinary English length of tube. 



Prazmowski (Hartnack) gives the following values for his 

 oculars and objectives : 



IMMERSION OBJECTIVES WITH CORRECTION. 



Zeiss, whose objectives have been much in request during 

 the past year or more, is noted for great working distance ; 

 his angles are low and he designates his objectives by 



