CENTRING ADJUSTMENTS 29 



very convenient for changing condensers, etc. On the 

 other hand, there is the fear that after some wear the 

 condenser will not return to its correct position unless 

 the fitting is very substantially built. 



Centration of Condenser and Objective. Some provision 

 for centring the condenser with the objective is necessary 

 for delicate work, and centring screws are, in many 

 cases, fitted to the substage ring or to the condenser 

 mount itself for this purpose. On many stands, however, 

 no centring screws will be found, and, in the absence 

 of a centring nose-piece, reliance must be placed on 

 the accuracy with which the maker has adjusted the 

 microscope. In such cases the centration of the con- 

 denser should always be carefully examined (page 145). 

 Unfortunate]\ r it leaves much to be desired on some 

 of the cheaper stands, and, however well it has been 

 carried out, it is only accurate for the condenser and 

 objective originally fitted, and the absolute necessity 

 of centring screws to the substage or nose-piece becomes 

 obvious when either condenser or objective is changed. 

 When a condenser is found to be slightly out of adjust- 

 ment in a non-centring substage, rotation in the ring 

 may bring it into more accurate alignment with the 

 objective. The absence of centring screws is often asso- 

 ciated with an Abbe illuminator, as that does not require 

 such accurate adjustment as an achromatic condenser. 



Some workers prefer to attach the centring screws to 

 the nose-piece which carries the objectives, claiming 

 the advantage that once the lamp, condenser, and ob- 

 jective have been accurately centred, any change 

 of objective requires only a slight adjustment of the 

 nose-piece, whereas it may be necessary to adjust the 

 lamp as well as the condenser to get them all in line again 

 if the centring screws are attached to the condenser 

 mount ; and objectives are changed more frequently 

 than condensers. 



