CH. IT] LIGHTING AND FOCUSING 47 



therefore almost in contact with the lower surface of the slide. For 

 some purposes when it is desirable to avoid the loss of light by reflec- 

 tion or refraction, a drop of water or homogeneous immersion fluid is 

 put between the slide and condenser, forming the so-called immersion 

 illuminator. This is necessary only with objectives of high power 

 and large aperture or for dark-ground illumination. 



§ 87. Centering the Condenser. — The illuminator should be 

 centered to the optic axis of the microscope, that is the optic axis of 

 the condenser and of the microscope should coincide. Unfortunately 

 there is extreme difficult}' in determining when the Abbe illuminator is 

 centered. Centering is approximated as follows : Put a pin-hole dia- 

 phragm — that is a diaphragm with a small central hole — over the end 

 of the condenser (Fig. 52), the central opening should appear to be in 

 the middle of the field of the microscope. If it does not the condenser 

 should be moved from side to side by loosening the centering screws 

 until it is in the center of the field. In case no pin-hole diaphragm 

 accompanies the condenser, one may put a very small drop of ink, as 

 from a pen-point, on the center of the upper lens and look at it with 

 the microscope to see if it is in the center of the field. If it is not, 

 the condenser should be adjusted until it is. When the condenser is 

 centered as nearly as possible remove the pin-hole diaphragm or the 

 spot of ink. The microscope and illuminator axes may not be entirely 

 coincident even when the center of the upper lens appears in the cen- 

 ter of the field, as there may be some lateral tilting of the condenser, but 

 the above is the best the ordinary worker can do, and unless the 

 mechanical arrangements of the illuminator are very deficient, it will 

 be very nearly centered. 



It is to be hoped that the opticians will devise some kind of 

 mounting for this the most commonly used condenser whereby it may 

 be centered as described for the achromatic condenser instead of by the 

 crude methods described above. If the condenser mounting regularly 

 possessed centering screws as in the microscope of Watson & Sons and 

 there were a centering diaphragm in the proper position so that its im- 

 age could be projected into the field of view, the operation would be 

 very simple. If, further, the condensers of Powell and Lealand were 

 selected as models the condensers need not be so bulky, and would still 

 retain all their efficiency. 



Fortunately the Royal Microscopical Society of London which has 

 done so much toward standardizing microscopical apparatus has recently 

 proposed as a standard size for the substage fitting for the condenser of 

 1.527 in. = 38.786 mm. (see § 46). 



