290 THE USE OF THE MICROSCOPE 



Spectrum. The overlapping series of colored images (real or virtual) 

 resulting when light has been dispersed by obhque incidence on a refrac- 

 tive surface, or by diffraction from a grating. 



Spherical aberrations. (1) Axial spherical aberrations due to the 

 spherical shape of the surfaces of the lens; the amount of refraction for 

 the outer zones of a convex lens being greater than for the inner zone. 

 (2) Aberrations for oblique pencils. (3) Differences of spherical aberra- 

 tions for different colors. 



Stage. The platform on which the slide is moved under the objective, 

 at right angles to the optic axis. A mechanical stage is such a platform 

 moved by rack and screws. An object traverser (detachable mechanical 

 stage) is an apparatus for mechanical motion of the slide over the fixed 

 stage, as is also a slide bar. 



Stain. A dye, usually selective, applied to tissues, or other micro- 

 scopic objects, with or without a mordant, and usually partially removed 

 by another reagent (differentiation). 



Standard screw. The ordinary objective screw, from the standard 

 of the Royal Microscopical Society. 



Steinheil lens. A triplet magnifier, first calculated by Steinheil. 



Stereoscopic. Showing views from slightly different angles in the 

 two eyes. In the monobjective binocular, light which forms the left 

 of the aperture pencils coming from points in the (virtual) reversed 

 image must enter the left eye, and vice versa, or the vision is pseudoscopic, 

 elevations appearing as depressions. The Greenough binocular is 

 the chief stereoscopic microscope, and has (and must have) erect images. 



Stop. (1) A circular central diaphragm, usually below a condenser, 

 for dark-field or annular illumination. (2) Any diaphragm. 



Submicroscopic. Smaller than the limit of resolution, so that only 

 the diffraction discs and diffraction lines are seen. 



Substage. Sleeve, with rack or screw, arranged to hold the 

 condenser. 



Test. An estimation of the optical capacity of the microscope with 

 any objective (which may have deteriorated). The star test is one of 

 the best (for spherical aberration). In this a minute hole in a silver or 

 nigrosin layer is used as object. Surirella gemma in hyrax is a good 

 test object. 



Total reflection. Reflection of all incident light within the more 

 refracting medium after exceeding the critical angle of incidence. 



Tourmalin. A natural crystal, which cut in a certain direction allows 

 only, or mostly, fight polarized in one plane to pass. It should be 3 

 millimeters thick (Beck). 



Translucent. Allowing some light to pass, but not usually enough for 

 vision; as in ground glass or opal glass. 



Transmission. Percentage of light passed through a transparent 

 substance, such as a colored screen. Usually different for the different 

 colors. 



