THE FUNDAMENTAL PRINCIPLES. 27 



What is meant by the focal distance of an objective? The working dis- 

 tance? 



What is the difference between the dry and immersion objectives? 



What is a homogeneous immersion objective? 



What is meant by a non-achromatic objective? 



What is meant by an achromatic objective? An aplanatic objective ? 



What is an apochroinatic objective? 



What is meant by an adjustable objective? 



What is the angular aperture of an objective? 



What is meant by the actual point of an objective? 



What is the optical axis of a microscope ? 



What relation does the size of the lenses have to its angular aperture? 



What is the numerical aperture of an objective ? What is the ocular of a 

 microscope ? 



Of how many lenses does it consist? 



What is the difference between the positive and the negative ocular. 



What is a compensating ocular? A projecting? 



How are oculars designated ? 



What is the field of a microscope ? 



What is the eye-point? 



What care should be given to a microscope? 



Describe the tests for determining the cause of an obscure image. 



Describe the methods of cleansing the lenses of the microscope. 



CHAPTER I. 

 THE FUNDAMENTAL PRINCIPLES. 

 THE HISTORY OF BACTERIOLOGY. 



WHEN in the latter part of the seventeenth century 

 Anthony von Leuwenhoek, by means of his magnifying- 

 glasses, first discovered organisms in decaying vegetable 

 infusions, he may be said to have laid the very first stone in 

 the foundation of what later on was to be the Science of 

 Bacteriology. 



It was very long after this, however, before sufficient facts 

 were collected to place this science upon a firm basis, and it 

 remained for a genius like the immortal Pasteur and the 

 eminent talents of the equally great Koch to build up the 

 superstructure of bacteriology so as to have it accepted by 

 all as the true basis of scientific medicine. 



When first observed, these microorganisms were supposed 



