OCULARS AND OBJECTIVES 55 



the Abbe. These improved condensers must be 

 absolutely centered to the objective and capable of 

 very accurate focussing otherwise much of their value 

 is lost. 



8. Mirrors. Below the substage condenser is at- 

 tached a gymbal carrying a reversible circular frame 

 with a plane mirror on one side and a concave mirror 

 on the other (Fig. 40, g). The plane mirror is that 

 usually employed, but occasionally, as for example 

 when using low powers and with the condenser racked 

 down and thrown out of the optical axis, the concave 

 mirror is used. 



9. Oculars, or Eyepieces. Those known as the 

 Huyghenian oculars (Fig. 48) will be sufficient for all 

 ordinary work without resorting to the more expen- 

 sive "compensation" oculars. Two or three, mag- 

 nifying the "real" image (formed by the objective) 

 four, six, or eight times respectively, form a useful 

 equipment. 



As an accessory Ehrlich's Eyepiece is a very useful 

 piece of apparatus when the enumeration of cells or 

 bacteria has to be carried out. This is an ordinary 

 eyepiece fitted with an adjustable square diaphragm 

 operated by a lever projecting from the side of the 

 mount. Three notches are made in one of the sides of 

 the square and by moving the lever the square aperture 

 can be reduced to three-quarters, one-half or one-quar- 

 ter of the original size. 



10. Objectives. Three objectives are necessary: one 

 for low-power work e.g., i inch, inch, or J inch; 

 one for high-power work e. g., -^ inch oil immer- 

 sion lens; and an intermediate "medium-power" lens 

 e. g., J inch or J inch (dry). These lenses must be 

 carefully selected, especial attention being paid to the 

 following points: 



(a) Correction of Spherical Aberration. Spherical 

 aberration gives rise to an ill-defined image, due to the 



