255 



V 



Some Remarks on the Interpretation of Microscopic Images 



with High Powers. 



By E. M. Nelson. 



(Read January 22nd, 1SS6.J 



The answers to biological questions, or the keys to Natural 

 History puzzles, as they may be appropriately called, are, 

 thanks to the improvements in the microscope, becoming more 

 minute every day. The inch and half -inch objectives have 

 unlocked many of Nature's hard enigmas, but as the ground is 

 being worked over by higher powers there is less left within 

 their range. 



It is with guns such as the wide-angled oil y 1 ^ that we must 

 pound away at Nature's citadels if we wish to capture her 

 strongholds. The interpretation of a microscopic image under 

 an inch or half-inch presents but little difficulty, but in the use 

 of lenses such as the wide-angled oil T ^-, two questions have to 

 be answered before you can satisfy yourself that your interpre- 

 tation of the image is correct. These are : (1) Is the object pre- 

 cisely in the focus of the objective ? (2) Is the lens in perfect 

 adjustment ? 



It is well known that slight variations in either focus or 

 adjustment, or both, will produce a marked effect on the 

 resultant picture. There cannot be, therefore, two more 

 important questions for the microscopist of to-day than : What 

 is focus ? What is adjustment ? 



The difficulty in solving these questions will depend largely 

 on the kind of object under examination. 



Bacteria, stained and mounted in balsam, form a class of 

 objects which, perhaps, offers fewer obstacles than any other to 

 the solution of these questions, hence they are suitable for test 

 objects. Even with these easy objects some discussion has 

 taken place with regard to the interpretation of the image. 

 There are those who say, that a certain bacillum consists of a 

 number of elongated spores within a cylindrical hyaline sheath ; 



