WORKING DISTANCE. 103 



reader is referred, while the introduction of a good series 

 of test objects renders the general employment of the 

 former scarcely necessary. 



It must not be forgotten that great differences exist 

 between test objects. Amongst the slides of Pleurosigma 

 angulatum, sold for this purpose, some are so extremely 

 coarse as not to be a test in any sense of the word, while 

 others are so finely marked that they can only be resolved 

 with the greatest difficulty under a J-inch objective of high 

 air angle. This is also the case with the diatoms Navicula 

 rhomboides and A mphipleura pellucida, used as tests for the 

 highest powers ; so that no reliance should be placed upon 

 statements that such and such a diatom was resolved 

 under a certain objective. 



Object-glasses for use with the microscope are usually 

 spoken of as possessing the following qualities : 



1. Working distance of the front lens from the object ; 



2. Defining power ; 



3. Flatness of field and freedom from distortion ; 



4. Penetrating power ; 



5. Resolving power ; 



and it is to ascertain their excellence, or otherwise, 

 in these directions that test objects are brought into 

 use. 



WORKING DISTANCE. It has already been shown that 

 the nomenclature of objectives does not presuppose any 

 working distance from the front lens ; in fact, such a thing 

 would be impossible, seeing that the enlargement of the 

 aperture reduces the distance of the front lens from the 

 object, while the amplification remains the same. Their 

 designation, such as a " i-inch objective," indicates only that 

 such an object-glass should possess the same magnifying 

 power as a single lens of i-inch focus, the distance the 



