Microscope Construction, Use, and Care 793 



object does not pass through the object but is rcflecied from ilie 

 surface of the object. None of the light from the illuminant reaches 

 the eye directly. The object thus appears to be self-luminous against 

 a black background. Illumination of the object is obtained by either 

 a standard condenser provided with an adapter or by means of a 

 special dark-field condenser. 



The simplest form of adapter consists of a wheel-shaped metal disk 

 inserted into the slot below the condenser. The center of the disk cuts 

 off the central rays of light and illuminates the object with the oblique 

 marginal rays. A more effective adapter is a unit that replaces the 

 upper element of the Abbe condenser. 



The much more expensive dark-field condensers are of two 

 principal types. Refracting condensers provide an oblique cone of 

 lioht by refraction through the marginal regions of the condenser 

 lenses. A disk below the central region of the condenser shuts out 

 light from that portion. Reflecting condensers produce an oblique 

 cone by total reflection from internal surfaces of the condenser lenses. 

 Diagrams and descriptions of the various types of condensers can be 

 found in the catalogues. 



Dark-field illumination is recommended for the study of filamen- 

 tous or unicellular algae and fungi, as well as for unstained sections of 

 tissues. The cytoplasmic strands and nuclei of Spirogym and cyto- 

 plasmic streaming in leaves of Elodea and filaments of Rhnojms make 

 striking and instructive demonstrations. 



The discussion of sources of light for the microscope has been 

 deferred to this point, where the source can be discussed in 

 conjunction with the condenser and the other optical components. 

 Illumination is said to be critical when the source of light is super- 

 imposed on the object. This means that if an unfrosted tungsten coil 

 bulb is the source, the coil is sharply defined upon the object. It is 

 true that the portions of the object that coincide with the coil are 

 under critical illumination, but only a very small part of the field 

 may be so illuminated, and it is obvious that a naked coil cannot be 

 used in this manner. 



A frosted bulb is some improvement, but the granularity of the 

 bulb is visible under critical conditions, as defined above, and the 

 curvature of the bulb is visible under lower powers. If the condenser 

 is lowered to obscure the granularity and cur\ature, the resolving 

 power is decreased. 



The desirable source is a flat, luminous, grainless surface of 

 sufficient size to cover the field of the lowest power objective. When 



