156 Animal Microloyy 



Professor Abbe, in 1873, proposed the name numcrirul <i/>crfure and 

 introduced the formula N. A..=n sin u in which n signifies the refractive 

 index of the medium between cover-glass and objective, and u equals 

 half the angle of aperture. That is, by multiplying the refractive index 

 of the medium by the sine of half the angle of aperture, the numerical 

 aperture is obtained. For example, suppose that one had an oil-immer- 

 sion lens of 90 degrees angular aperture, then half the angle of aper- 

 ture is 45 degrees, and by turning to a table of natural sines, the sine of 

 45 degrees is found to be 0.707. The refractive index of cedar oil is 1.52. 

 Then N. A. =1.52x0.707=1.075. Suppose that the lens were a dry instead 

 of an immersion lens; then since the refractive index of air is 1, the for- 

 mula would read N. A. =1x0.707=0.707. Thus the two products 1.075 

 and 0.707 respectively, represent the relative capacities of an oil immer- 

 sion and a dry objective of 90 degrees angular aperture. 



Parfocal. A term ordinarily applied to eyepieces of different powers 

 that may be exchanged in the microscope without very materially affect- 

 ing the focus of the instrument. The term is also applied to objectives 

 attached to a revolving nosepiece if each is approximately in focus when 

 turned into place. 



Pedesis. Same as Broivnian movement. 



Penetration. The quality of an objective that permits of "looking 

 into" an object having sensible thickness. It is greatest with low powers 

 and narrow angles and is antagonistic to resolving power. It is the nat- 

 ural consequence of certain conditions in the making of lenses and is 

 reckoned of secondary importance, because practically the same results 

 are obtained by manipulating the fine adjustment. 



Polariscope. As used in microscopy the polariscope consists of two 

 parts, each composed of a Nicol prism of Iceland spar; one, the polar- 

 izer, fits into the substage, and the other, the analyzer, is inserted between 

 the objective and the tube of the microscope or, in some forms, just above 

 the ocular. The polariscope is used more in chemical and in geological 

 than in histological work. Some of the uses are as follows : determining 

 whether an object is singly or doubly refractive; detecting the presence 

 of minute crystals; determining the composition of rocks; examining 

 sections of bone, hoof and horn, hairs and fibers of animals and plants, 

 starch, etc., for certain characteristic and striking effects. 



Resolving Power. The quality of an objective which enables the 

 observer to make out fine details of structure. It is the most essential 

 property for precision in observation, and determines largely the excellence 

 of an objective. Resolving power depends upon careful correction of 

 aberrations, general accuracy in the mechanical construction of the 

 microscope, and upon the aperture of the objective (see aii//ul<ir <ijicrture, 



