MICROSCOPICAL (ILOSSARY 289 



Radiant. The source of light; incandescent surface, or illuminated 

 ground glass. 



Ramsden circle. The exit aperture pupil of the microscope, the 

 largest circle abo\-e the eyepiece. It may be magnified to determine the 

 aperture of the condenser by a Steinheil, 10-times magnifier; by a 

 15-times compensating eyepiece; or by an inverted Huyghenian eyepiece. 



Ramsden eyepiece. With diaphragm below two simple lenses. 

 Rarely used. 



Reflection. The change of course of light by an interposed surface, 

 which the light does not penetrate, but makes equal angles on each side 

 of the perpendicular. (Total reflection occurs only in the medium of 

 greater refractive index, and when the angle of incidence is beyond 

 a certain amount [about 40 degrees for glass and air].) 



Refraction. The change of course of light which penetrates an inter- 

 posed surface more or less obliquely. It is measured by the ratio of 

 the perpendiculars on the normal from unit distance on the incident and 

 the refracted ray (sines of the angles) . Refraction is different for light 

 of different frequencies of waves. 



Refractive index of any substance (R.I.) is the ratio of the perpen- 

 dicular distances of the ray, before and after refraction, from the normal 

 to the surface (sines of the angles). The substance is presumed to be 

 in vacuo. 



Resolution of fine gratings. The rendering visible, or photographing, 

 of the correct number of close lines (without regard to their thickness). 

 Resolution is directly proportional to working aperture, and inversely 

 proportional to wave length. 



Searcher, finder. A low- or medium-power objective on the nose- 

 piece, used in the systematic examination of a preparation. 



Semi-apochromatic. (1) Sometimes applied to objectives with 

 improved color corrections (Spitta); or (2) to fluorite objectives, with 

 the secondary spectrum diminished. 



Sharpness. Absence of fog or glare, and freedom from blurring due 

 to spherical aberrations. 



Slides. Slips of good plate glass with ground edges and corners, 

 usually 3 inches by 1 inch. Their thickness should be uniform. The 

 writer prefers them 1 millimeter thick. They should be freed from 

 grease below, before using them with a water-immersed condenser. 



Solid cone. Light of all apertures focusing simultaneously at a point 

 on the ol)ject, and requiring the absence of axial spherical aberration 

 in the condenser. 



Source -field. That area of the primary or secondary source of light 

 whose image formed in the plane of the object equals the object-field. 

 For good vision, the source of light should be not greater than the source- 

 field, and for ultimate resolution the source should b(; a small central 

 spot in the source-field. Hence a series of diaphragms, or an iris, is 

 needed, close to the light. 



