280 HIGH POWER MICRO-PROJECTION [Cn. IX 



entire beam of light from the lamp condenser may enter. Then, 

 just as in ordinary observation, one can often make the contrast 

 more striking by cutting down the aperture somewhat by closing 

 more or less the substage condenser diaphragm. It must not be 

 cut down too much, for that will render the image dim and defeat 

 the very purpose of the substage condenser. 



As a general statement, much more of the aperture of the 

 objective can be used in projection than in ordinary direct observa- 

 tion in the microscope. Naturally, objectives of relatively large 

 aperture give the more brilliant images (see 855). 



405. Oculars to use in projection. Generally speaking, only 

 low powers arc used (x2, X4, x8). The lower the power the more 

 brilliant the image. Compensation oculars have been found better 

 than the Huygenian. A compensation ocular as high as xi2 gives 

 brilliant images for short screen distances. 



One should not forget that the ocular, when used in projection, 

 is really a second projection system, and hence the image will be 

 erect on the screen (fig. 207). 



404a. Centering the substage condenser. As the substage condenser 

 becomes one of the optical elements in projection, its' principal optic axis must 

 be centered on the common axis of the entire apparatus. 



It is assumed that the microscope without the substage condenser has been 

 properly centered as directed in 374-375. 



To center the substage condenser, use the ocular and objective (x4 ocular, 

 8, 10 or 1 6 mm. objective), remove the bellows if present (fig. 133), place a piece 

 of white cardboard at about 45 degrees as shown in fig. 1 16, between the large 

 condenser and the substage condenser, and light the cardboard well with a 

 mazda lamp. This will give the light for the microscope. 



Now put a preparation on the stage and focus the microscope as for ordinary 

 observation. Remove the specimen and close the substage iris diaphragm 

 nearly up. With a pocket magnifier examine the eye-point or Ramsden's disc 

 (fig. 127 E P) beyond the ocular. This disc of light appears as if on the back 

 lens of the objective. If the iris is properly made and the substage condenser 

 is centered with the objective and ocular, the center of light will appear to lie 

 exactly in the middle of the back lens of the objective (fig. 151). If the sub- 

 stage is not in the optic axis then the disc of light will appear eccentric; and 

 if the substage condenser is markedly off the center the spot of light will make 

 a break in the black ring on one side as shown in fig. 30, 1-4. If it is only 

 slightly off center, the disc of light will seem to be surrounded by a dark ring 

 of unequal width. If the substage condenser is not found to be correctly 

 centered, the centering screws (fig. 150) must be used to move it slightly until 

 the disc of light is central as shown in fig. 151. 



The Abbe condenser found on most microscope's has no centering screws. 

 Tlu' makers center the instrument carefully and fix it in position. If it is 

 found badlv out of center it is best to return it to the makers for adjustment. 



