CH. II] 



LIGHTING AND FOCUSING 



43 



Fig. 41. Zeiss' Achromatic Conden- 

 ser, c. s. c. s. Centering screws for 

 changing the position of the condenser 

 and making its axis continuous with 

 that of the microscope. A segment of 

 the condenser is cut away to show the 

 combinations of tenses. For very low 

 powers the upper lens is sometimes 

 screwed off. There is an iris dia- 

 phragm between the middle and lower 

 combinations. (Zeiss' Catalog). 



§ Si. Centering the Condenser. — To get the best possible 

 illumination for bringing out in the clearest manner the minute details 

 of a microscopic object two conditions are necessary, viz.: The princi- 

 pal optic axis of the condenser must be continuous with that of the 

 microscope (see frontispiece) and the object must be in the focus of the 

 condenser, i. e., at the apex of the cone of light given by the condenser. 



The centering is most conveniently accomplished as follows 

 although daylight may be used with almost equal facility. A very 

 small diaphragm is put below the condenser. (If the Zeiss achromatic 

 condenser is used, the diaphragm of the Abbe illuminator serves for 

 this. If there is no pin-hole diaphragm one can be made of stiff, 

 black paper. Care must be taken, however, to make the opening ex- 

 actly central. This is best accomplished by putting the paper disc over 

 the iris or metal diaphragms and then making the hole in the center of 

 the small circle uncovered by the metal diaphragm. For the hole a fine 

 needle is best). Light well and lower the objective so that it is at 

 about its working-distance from the top of the condenser. If now the 

 condenser is lowered or racked away from the objective the image of 

 the diaphragm will appear. If the opening is not central it should be 

 made so by using the centering screws of the condenser. 



A better plan than to lower the condenser to focus the image of the 

 diaphragm, is to raise the body of the microscope slowly with the coarse 

 adjustment. It is almost impossible to make apparatus so accurate that 

 two parts like the body of the microscope and the substage, each work- 

 ing on different sliding surfaces, shall continue in exactly the same 

 plane. So one will find that if the condenser be accurately centered 

 with the condenser lowered, and then the condenser be racked up close 

 to the stage and the image of the diaphragm opening brought again 

 into focus by racking up the body of the microscope, it will not be 



