THE MICROSCOPE 41 



surface of the slide; otherwise much light will be lost 

 by refraction, in passing from the condenser to the 

 object. 



The iris diaphragm is usually included in the con- 

 denser mounting. Although this is a mechanical 

 device, it is so intimately connected with the working 

 of the optical parts that it should be considered in 

 conjunction with these. Ordinarily an excessive 

 amount of light is projected upon the object by the 

 mirror, and the image appears blurred. The iris 

 diaphragm, by regulating the amount of light reaching 

 the object, reduces this excessive illumination. While 

 proper adjustment of the diaphragm opening is an 

 important factor in securing clear-cut views, no abso- 

 lute rule can be given for this adjustment. The size 

 of the diaphragm opening varies with the strength 

 of the light, the different combinations of objectives 

 and oculars, the density of the object and the vari- 

 ations in the eye of the observer. Aside from the 

 general rule that one must use larger diaphragm open- 

 ings with higher power objectives, the proper adjust- 

 ment of the diaphragm under different conditions 

 must be learned by experience. 



MECHANICAL PARTS OF THE COMPOUND MICROSCOPE 



While advances in histology, oacteriology and the 

 allied sciences have been coincident with the per- 

 fection of the optical parts of the compound micro- 

 scope, improvements in the mechanical parts of the 

 instrument have played an important part. High- 

 power lenses demand extreme accuracy in focusing 

 and firm, yet readily adjustable, support; therefore 



