288 THE USE OF THE MJC'EOSCOPE 



Oil-immersion olijectivcs have n dro]) of thickened redar oil, of 

 refractive index 1.515 for the D line, between front lens and cover. 



Opal glass contains a milky deposit. Flashed opal glass has a thin 

 layer of milky-white glass on a sheet of transjiarent glass. If it trans- 

 mits enough light, which is not usual, opal glass may be used like ground 

 glass, as a secondary source of light for the microscope. 



Opaque illumination. This may be effected: (1) by a bull's eye; 

 (2) by an annular lamp; (3) by a concave annular glass reflector (Beck) ; 

 (4) by vertical illumination through the objective. This last method 

 may well be applied to ordinary objects in a dark field with high-power 

 oil-immersion objectives. 



Optic axis. Line through center of the condenser, objective, eye- 

 piece or instrument, including, as nearly as possible, the centers of all 

 lenses. 



Optical tube length. Distance from posterior focal plane of objective 

 to anterior focal plane of eyepiece. 



Orthoscopic eyepiece, of the magnifier form, with triplet and single 

 eyelens. 



Paraboloid. A short truncated glass reflector used for dark-field 

 illumination. 



Paraxial. Indefinitely close to the optical axis. 



Periplane eyepiece. Has compound eyelens with a field lens, and 

 gives a flatter field (Metz). 



Penetration. Depth of vision through lens or microscope without 

 altering adjustments of the microscope. Mostly due to accommoda- 

 tion of the eye. 



Plane -parallel glass has its surfaces made optically true and parallel, 

 by grinding and testing. 



Polariscope. Arrangement, usually of two Nicol prisms or two 

 tourmalins, of which one polarizes the light, and the other stops most 

 or all of this polarized light, showing objects which alter the polarization 

 bright in a dark field. 



Polarized light. Produced or isolated by one Nicol prism, or one 

 tourmalin, and used at the right azimuth to improve definition of fine 

 markings (Stump, 129). 



Porro prisms. Usually two right-angled prisms crossed, in each of 

 which the light (of nearly plane waves) undergoes two total reflections. 

 The image is in consequence, inverted. In the compact form, used in 

 the ordinary Greenough, these prisms are combined on each side. 



Presbyopy. State of the eye due to increased age, in which the power 

 of focusing the lens is gradually lost, so that only distance vision remains. 

 It is aided by convex reading glasses. 



Prism. In optics, a body usually of glass bounded by inclined planes. 

 Prisms are usually regarded as dispersing the colors, but a reflecting 

 prism of isosceles section is achromatic. 



