is more delicate and one which must be so faultless. 

 It will be noticed from the diagram that objectives of 

 different power are different in construction and this is 

 also true of objectives of the same power but different 

 in efficiency. Objective makers follow different lines 

 to obtain results, and as a large variety of glass is now 

 offered from which to select, widely different construc- 

 tions follow, which in objectives of the same class from 

 reputable makers are very similar in efficiency. 



The optical glass used must be free from defects 

 and must be absolutely permanent under all climatic 

 conditions. This glass comes in thick plates from the 

 maker and is sawed into slabs of suitable thickness 

 from which are cut pieces of the requisite size for 

 lenses. These are then cut into lens form and the 

 various processes of grinding, polishing, and edging 

 follow. The control of work in these various steps 

 must be absolute. There must be no pit holes or 

 scratches, no variation in thickness or diameter, and 

 the surfaces must be so perfect that all mechanical 

 means of measurement fail and recourse must be had 

 to highly sensitive optical tests to determine errors. 

 The difficulties increase with the increase in the power, 

 as all defects are magnified to the extent of the magni- 

 fying power of the microscope, and furthermore the 

 difficulties of handling and manipulating the lenses 

 become more serious as the lenses become smaller. 

 The lenses are then mated and cemented, set and fixed 



6? 



