its upper surface. With the lowest powers a condenser 

 is not needed, but for the medium and higher powers the 

 condenser not only furnishes the amount of light needed, 

 but provides an easy means of providing each objective 

 with a cone of light suitable to its aperture. 



Condensers are of two great classes the archomatic 

 and non-achromatic. The achromatic are much the 

 better, and are indispensable for photo-micrographic 

 work, but as the non-achromatic (Abbe) is in such general 

 use on account of its lesser cost and because it is suffi- 

 cient for ordinary work, we will consider it especially. 

 The same general rules apply to both, which are funda- 

 mentally the same as those given before, with the excep- 

 tion that in most cases the plane mirror is used because, 

 as stated above, the condenser is made for parallel rays 

 of light. 



After removing the ocular, turn the mirror so that the 

 back lens of the objective is fully and evenly illuminated 

 and, if possible, free from any images of trees, window 

 sash, etc. If these images cannot be dispelled by turning 

 the mirror, use the concave mirror. Slightly lowering 

 the condenser will also accomplish the end. There is an 

 objection to both of these methods, which will be ex- 

 plained later. 



When this is accomplished the proper cone of light 

 must be secured by opening or closing the diaphragm 

 below the condenser. 



A good general rule is to close the diaphragm so that 

 in looking at the back lens of the objective the diaphragm 

 opening, which can be plainly seen, appears to be about 

 half the diameter of the back lens of the objective when 

 it is in focus. Then with the ocular in place change the 

 opening to give the best results. The thinner the tissues 

 and the greater the contrasts the larger the cone of light 

 which may be used. Thicker tissues and those with less 

 19 



