212 OBJECTIVE AND SUBSTAGE CONDENSER TESTING 

 Periphery of Field. 



A straight vertical line indicates perfect lens in correct focus, 

 (b) An inclined straight line shows perfect lens in incorrect focus. 

 If the inclination is different to that found at the centre, the 

 difference shows the degree of curvature of field. 



(c) A bent line denotes presence of aberrations: — disobedience of 

 sine conditions, etc. 



Experience shows that " performance curves " for the centre of 

 the field show almost at a glance the aberrations present, and their 

 degree. Interpretation of curves for the periphery of the field is 

 more difficult. 



Colour filters only have so far been used for obtaining approxi- 

 mately monochromatic li^ht, a prismatic spectral illuminator would 

 be a valuable addition. 



It will be observed that this method of objective testing has been 

 developed from the method of adjusting tube length described in a 

 previous paper. (3) 



(3) Hartridge, Journ. Roy. Micro. Sac, 1919, p. i ig. 



