282 THE USE OF THE MICROSCOPE 



Crown glass. Usually contains no lead, but lime and alkali with 

 silica. 



Curvature of field. Cannot be corrected in the objective. Most 

 conspicuous with low eyepieces. Reciuires, for correction in photog- 

 raphy, concave lenses, such as the homal projection lenses (Boegehold). 



Dammar. Resin from a Southern conifer. The colorless lumps are 

 selected, and a little xylol added. The thick solution is decanted after 

 a few days. It is useful for sealing iron-acetocarmine preparations. 



Dark field (dark ground). Illumination by beams of light which 

 pass outside the aperture of the objective. (Dark field may also 

 result from vertical illumination through the objective.) With dry 

 objectives and a dark-field condenser, much light may also be reflected 

 from the upper surface of the cover-glass on to the object. 



Definition. Sharpness, brilliancy and contrast in the image. Due, 

 ceteris paribus, to good corrections for spherical and chromatic aberra- 

 tions. Good centering is also necessary. 



Depth of vision (penetration). Vertical distance within which 

 definition is passably good. Mostly due to accommodation of the eye. 

 Usually slight above 100 magnification. 



Diaphragm. Usually a blackened stop with a circular aperture. 



Dififraction. Regular scattering of light in definite directions from the 

 edge of a body, or from small bodies. It increases as the size of these 

 decreases and approaches the wave length of the illumination, and is 

 greater with shorter wave length. Submicroscopic bodies are seen by 

 diffracted light. The intensity of this is proportional to the sixth power 

 of their diameters (Rayleigh). A series of evenly spaced lines or dots 

 causes a summing up of the diffractions at the apertures into definite 

 diffraction beams. To give the best diffraction beams, light should 

 fall on a submicroscopic line in an azimuth at right angles to the line 

 (Abbe, Siedentopf, Conrady). The diffracted beam may be polarized 

 at high angles, and then can be freed from useless light by a tourmalin 

 or a Nicol prism, at the correct angle, before the source of light. 



Diopter. Optical strength (reciprocal of focal length) of a lens whose 

 focus is 1,000 millimeters. The unit for low-power lenses. 



Dispersion. Differences in relative refraction of differently colored 

 lights (of different wave lengths) in different transparent substances. 

 Measured by the ratio of the refraction of violet minus that of red to that 

 of yellow. 



Doublet. Two combined lenses, usually cemented. 



Drawtube. For lengthening or shortening the optical tube length, 

 and consequent adjustments of objectives for different thicknesses of 

 cover-glass or mounting medium. 



Dry objective. One calculated for air between front lens and cover- 

 glass. 



Error. Deviation from what is required (aberration). 



