PART V. 



THE SIMPLE MICROSCOPE AND THE 

 LANTERN MICROSCOPE. 



THE compound dioptric Microscope now takes unquestionably the 

 first place amongst the optical instruments employed for the 

 observation of small bodies. It has become the special instrument 

 of the naturalist, the chief weapon of the microscopist. The sphere 

 of action for its former rival, the Simple Microscope, grows yearly 

 less and less, and the Lantern Microscope (called, according to .the 

 kind of illumination adopted, Solar, Gas, or Photo-electric Micro- 

 scope) has been of little or no importance to science hitherto. The 

 reader will under these circumstances consider that we are justified 

 in devoting only a relatively short supplementary discussion to 

 these instruments. 



I. 



THE SIMPLE MICEOSCOPE. 

 A. General Principles. 



EVERY lens or combination of lenses, which is applied in such 

 a manner that the eye directly observes the virtual image formed 

 by it, may be regarded aspn principle a simple Microscope. In 

 practice, however, only the higher magnifying-lenses, or systems of 

 lenses, which for more convenient use are attached to a stand, are 

 usually included under this expression, while the low-power lenses 

 which can be held in the hand are termed magnifying-glasses. It 

 is obvious that this distinction must be somewhat arbitrary, since 



