OBJECTIVES. 23 



into the end of the tube H. The triple nose-piece E is a con- 

 venience for sliding one or the other objective into place as de- 

 sired. The stage is perforated in the center for transmitting 

 light, reflected up by the mirror O. In the opening there may 

 be fitted little cylinders with smaller openings, known as 

 diaphragms, the object of which is to regulate the amount of 

 light. There is a series of three or four of these. On the stage 

 are two clips for holding a glass slide, on -which the object is 

 examined. 



The iris diaphragm Q is much more convenient than the 

 cylinder diaphragms, as the opening can be made gradually 

 larger or smaller by simply turning a small lever back or forth. 



If a greater concentration of light is desired than is produced 

 by the concave mirror O, a condenser is used, which is placed in 

 position beneath the stage. The best form is the Able type 

 (Fig. 23), consisting of one lens or a system of lenses for con- 

 verging a large beam of light. The condenser is used for great 

 magnification and is invaluable in studying stained specimens, 

 which are to be differentiated by color rather than by outline. 



Condenser of 1.20 num. apert. Condenser of 1.40 num. apert. 



Fig. 23. 



ILLUMINATION. No fixed rule can be laid down in regard to 

 the size of opening in the diaphragm to be used for any given 

 magnification, as the amount of light to be passed through a 

 specimen depends somewhat on its nature and thickness. As a 

 general rule, large diaphragms are used for low powers, with 

 weaker illumination and small ones for high powers with strong 

 illumination. Weak illumination is brought about by the 

 plane mirror, stronger by the concave mirror and the use of a 

 condenser if desirable. Actual laboratory practice is better 

 than many words in teaching the student what is the best 

 illumination of an object. 



OBJECTIVES. The objectives are the most important parts of 

 the whole microscope. Instead of one lens they consist of a 

 system of two, three or four lenses, some of which are simple, 

 others compounded of a convex crown lens and a concave flint 

 lens, as described under chromatic aberration. The front lens 

 of the system always has a plane face which is turned towards 

 the object, and is usually a simple lens (plano-convex). Such 



