16 A MANUAL OF BACTERIOLOGY 



means of a camera lucida from the instrument, 

 whilst with one eye he looks at the object, and 

 with the other draws the figure. A very little 

 reflection will convince any one how desirable it 

 is to keep the nerves of the eye as nearly in their 

 right position as possible ; for an undue strain is 

 caused if they are strained, and the sight is injured 

 if much work is done. 



The object-glass or objective (Fig. 3 A) is the 

 most important part of the microscope ; conse- 

 quently it is necessary to have good lenses to do 

 satisfactory work. The objectives are known as low 

 and high powers but for bacteriological work, the 

 microscope should be provided with the following 

 objectives (Zeiss') : 



D and E (dry lenses), J (water immersion), ^ 

 (oil immersion). Zeiss' lenses give perfect defini- 

 tions, and everything there is to be seen can be 

 made out with the highest powers. Oil-immersion 

 lenses are taking the place of water high powers, as 

 they need no correction for the thickness of the 

 cover-glass, and are therefore much easier to use ; 

 1 the only drawback is that the essential oil (e.g. cedar 

 oil) used will dissolve Canada balsam, Dammar 

 varnish, and many of the sealing fluids, and it is 

 necessary to cover them with Hollis' glue, which is 

 not acted on by cedar oil.' Of Zeiss' high powers 

 the ^ oil-immersion lens is the best, and may be 

 thoroughly recommended for bacteriological research. 

 Oil-immersion lenses possess far greater brilliancy 

 and definition than the water and dry lenses [such 

 as Zeiss' K and L (water), and F (dry)]. In using 



