Appendix A : The Microscope and Its Optical Principles 145 



The objectives of French and German instruments are rated 

 in millimeters and the conventional distance of vision taken as 

 250 millimeters. An objective of 3 millimeters focus, therefore, 

 yields an initial magnification of 83.3 diameters (^ X 250 = 83.3) . 

 Compensating oculars (see below) bear numbers which indicate 

 the number of times the eyepiece, when used at a given tube- 

 length, increases the initial magnification. Ocular 12, for exam- 

 ple, with a 3-millimeter objective would yield a magnification of 

 83.3X12 = 1,000 diameters, with a standard length of tube. 

 Unfortunately this simple system does not apply to most ordinary 

 oculars which are more or less arbitrarily lettered or numbered. 



SOME COMMON MICROSCOPICAL TERMS AND APPLIANCES 

 (Alphabetically Arranged) 



Achromatic Objective. An objective corrected for chromatic aberration. 

 The correction is not absolute. 



Achromatism. Freedom from chromatic aberration. 



Angular Aperture. The angle (measured in degrees) formed at the 

 point of focus (F, Fig. 59) by the outermost rays (a F, b F) which trav- 

 erse the objective to form an image. This angle is an important con- 

 sideration because on it depends in large measure the 



defining or resolving power of the objective. It is evident /_ A , 



that the larger the angle is, the greater the number of rays 



V 



of light that will be admitted from an object. Thus the 

 object will be better defined to the eye. In low powers 

 the angle may be very wide, in high powers it must F 



necessarily be small. Two objectives, even though they FIG. 59. 

 may possess different powers of magnification, will have the same bril- 

 liancy if they are of the same angular aperture; on the other hand, if 

 they have the same magnifying power but differ in angular aperture, 

 the brilliancy is reduced in the one of smaller angle. In immersion 

 lenses the liquid used between the lens and the object, by reducing 

 refraction has the effect of increasing the angle of aperture. See 

 immersion objective, also numerical aperture. 



Apertometer. An instrument for measuring both the angular and the 

 numerical aperture of objectives. It is fitted to the stage of the micro- 

 scope. 



Aplanatism. Freedom from spherical aberration. The result is a flat 

 field as viewed through the microscope. Aplanatic lenses are usually 

 also achromatic. 



