2V 



greater capacity, it is with the understanding that it 

 is so constructed as to give this result. 



While many immersion objectives are constructed 

 to work both as dry and immersion, such a plan can- 

 not be said to be advantageous. Such objectives may 

 be made to work well in one direction and be of 

 indifferent quality in the other, or may be of medium 

 grade both ways. There is no question that the best 

 plan .is to have each objective selected with a view to 

 a specific purpose and use it for this purpose only. 

 There are two fluids in general use at the present 

 time, water and homogeneous fluid. The latter expres- 

 sion means of the same kind, and refers to the fact 

 that the fluid has about the same refractive and dispers- 

 ive power as glass, so that when this fluid fills up the 

 space between the two surfaces of glass, a ray of light 

 passes through the three mediums as if they were one 

 body. 



The two large classes of dry and immersion objec- 

 tives may again be subdivided into two classes objec- 

 tives for long and short standard tube. As followed 

 by some firms at present and what it is hoped will 

 become a universal custom in time, each objective is 

 marked for the tube-length for which it is corrected 

 and with which it is assumed it will accomplish the 

 best results. 



Objectives are sometimes called/<?7t'^;-.r, and in this 

 sense are divided into three classes : low, medium and 

 high. Dr. Carpenter classifies them as follows : low 

 powers, 3 inch, 2 inch \\ inch, 1 inch, f or J inch ; 



