152 the president's address. 



fulfil, as this will show the difficulties the ambitious computer 

 has to face. 



.A. An ordinary low-power achromatic must be corrected for, — 



(1) Primary spherical aberration ; 



(2) Elimination of coma ; 

 (.">) Primary colour. 



B. An ordinary high-power achromatic, in addition, must be 

 approximately corrected for, — 



(4) Secondary spherical aberration (spherical zones) for the 



preferred colour. 



C. Semi-apochromatics, besides fulfilling the above four con- 

 ditions in a highly perfect manner, must be made free from, — 



(5) Primary spherical aberration for a second colour (and 



nearly for all colours) ; and 



(6) Should be computed so as to give equal magnifications 



for all colours when used with compensating eye- 

 pieces : whilst 



D. The full apochromatic must further show, — 



(7) Freedom from the secondary spectrum. 



One word more and my story is told. In carrying out these 

 corrections it is essential to bear in mind that the aberrations 

 should be corrected in the order given ; an objective must 

 be free from the first three to be entitled to the description 

 of an achromatic one, from the first five to become a semi-apo- 

 chromatic, and from all seven to merit the true application of the 

 term apochromatic. 



Tourn. Queh it Microscopical Club, So: 2, Vol. IX., No. 56, April 1905. 



