278 THE USE OF THE MICROSCOPE 



over a wider extent of tlie image. The latter are sometimes called 

 aplanatic achromatic. (Metz.) 



Achromatic objectives, ^^'itll the spectral colors C and F usually 

 coinciding in focus. The spherical correction is for one color (yellow- 

 green). When improved by the use of new glasses, they approach 

 semi-apochromatics. 



Adjusting condensers. Compensating the condensers, by altering 

 distance of lamp, or thickness of slide, or screwing out top lenses, 

 or centering an achromatic converging lens below (Hartridge, Ainslie^ 



Adjusting objectives. Correcting them, either by a correction collar, 

 or by altering tube length (Nelson, Coles). 



Adjusting tube length. Correcting slight spherical aberrations due 

 to cover-glass thickness by lengthening (or less often shortening) the 

 tube length (Nelson). 



Amplification. Magnification. 



Amplifier. A concave corrected lens used at a certain distance behind 

 the objective to increase the tube length, as in the binocular Bitumi 

 attachment, without shifting the focus and thereby causing errors ; or to 

 flatten the projection field, as in the homal (Boegehold). 



Annular illumination. Narrow hollow cone from stopped condenser 

 showing bright ring at edge of back of objective. Its aperture should 

 not exceed that of the objective. Gives oblique illumination in all 

 azimuths. Useful only for fine details on more or less transparent 

 objects. (Conrady.) 



Apertometer. Graduated glass semicircle for measuring apertures 

 of objectives (Al)be, Zeiss). 



Aperture. The size of the cone of light concentrated on any point 

 of an object by the condenser, or receivable from any point of the object 

 by the objective, the two being usually different. The aperture is 

 measured by the length of the perpendicular to the axis from unit 

 distance on the edge of the cone. It is obvious that if the object is not 

 at the apex of the cones of light concentrated by the condenser, the 

 aperture is lessened. Also if all the cones do not have their apices on 

 the visible part of the object, part of the field is viewed with lower 

 aperture. Hence arise the advantages of correcting the condenser both 

 for spherical and chromatic al^errations. Aperture is also the sine of 

 the air angle of incidence of (usually) the extreme rays on the plane of 

 the front of a lens or combination (objective) ; or vice versa, at the back of 

 a lens or combination (condenser). It is often called, after Abbe, numer- 

 ical aperture (N.A.). If the rays are not in air, this figure is to be 

 multiplied by the refractive index of the immersion fluid (Abbe). (Note 

 that the words "front and back" are used here with regard to the 

 direction of the light.) 



Aperture, wasted. By using too low eyepieces (Abbe). 



