34 



results may be obtained with the former than with the 

 latter. 



Prof. Abbe has introduced a new mode of deter- 

 mining 'and naming aperture. He calls it Numerical 

 Aperture, and this expression is now generally 

 adopted. Thus, 1.0 of numerical aperture is equal 

 to 180 degrees in air or about 82 degrees in homo- 

 geneous immersion, and has a direct relative value to 

 the resolving power of the objective. A complete 

 aperture table may be found in the Proceedings of 

 the Royal Microscopical Journal and larger works on 

 the microscope. 



Achromatism. As has been stated before, when 

 single lenses are made to give a high magnifying 

 power, the chromatic and spherical aberrations pre- 

 vent corresponding advantages ; and as the objective 

 gives the image which is magnified by the eye-piece, 

 it is evident that if they exist in it, they are increased 

 by the ocular, and that especial care must be given to 

 exclude all faults as much as possible from it. Even 

 with the use of flint glass, it is impossible to free the 

 objective entirely from color ; there will remain a 

 residue of green and purple, and these colors will 

 fringe the object. These are called the secondary spec- 

 trum, and their presence in an objective is usually 

 evidence of the highest correction. 



The amount of color in an objective depends some- 

 what upon the power of the eye-piece, and becomes 

 more visible as a higher power is used. -Color out- 

 side of the secondary spectrum is not always preju- 



