PRACTICAL HISTOLOGY 



NOTES. 



High power objectives are adjusted either for a tube 160 mm. 

 (6J inches) long (continental length) or for a tube 10 inches long 

 (usually called English length ; it is however now only made for 

 special purposes). The definition of the objective suffers the more 

 the proper length is departed from. Thus if a nose-piece is used, 

 a correction in length is required, as by cutting down the brass mount 

 of the objectives. 



Objectives are generally named according to their equivalent focal 

 distance, as |, inch: 12mm., 4 '3 mm. and so onl An objective 

 consists of more than one lens ; its equivalent focal distance is the 

 focal distance of a single lens which has the same magnifying power 

 as the objective, so that lenses of the same equivalent focal distance 

 have the same magnifying power. 



The angle of aperture is the angle which the outermost rays, 

 passing from a point and through the outermost edge of the lens, 

 form with one another. The greater the angle the more the object is 

 seen from all sides. 



The numerical aperture = n sin u, where n is the index of refraction 

 of the medium in front of the objective (as air, water, or oil), and u 

 is the sine of half the angle of aperture. Other things being equal 

 the brightness of the image and the definition of the object increases, 

 the focal depth decreases, with increase of numerical aperture. 



Achromatic objectives and Huyghenian eye-pieces were, till recently, 

 the only ones ordinarily used. The achromatic objectives made 

 with the new Jena glass are sometimes called semi-apochromatic 

 objectives. 



Apochromatic objectives and compensating eye-pieces form a more 

 perfectly achromatic system than the foregoing. In the apochromatic 

 len' , three colours are brought to a focus instead of two, and there is 

 cor olete colour correction for all zones instead of for one only. 

 With any one objective and eye-piece in this system, the object is 

 very nearly in focus with all the other eye-pieces. 



Homogeneous oil-immersion objectives may be either achromatic or 

 apochromatic, they can be made with a greater numerical aperture 

 than the other objectives. 



The initial magnifying power of an objective is the magnification 

 which it gives at the distance of distinct vision (250 mm. and 10 inches) 



