OPTICAL THEORY OF LIGHT MICROSCOPE 



lenses it is possible to get better color correc- Illuminating Systems 

 tion in an objective. The semi-apochromat 

 uses fluorite to a limited extent, and repre- 

 sents a compromise between the achromat 



Brightfield Illumination. Most micros- 

 scopy is done with "brightfield illumination" 

 in which the illuminating beam is a solid 



and apochromat in performance. The apo- ,. ,. , , , 



, , ^ a ■. ^ ■ , • cone oi light concentrated on the specimen 



chromat uses several nuorite lenses in combi- , , , i i- , 



,. -.u 1 1 XI- by a condenser lens system, mounted directly 



nation with glass lenses, to achieve a very , , , • m, • i 



,.,, c ,. ^ ,. » beneath the specimen, ihis lens system is 



high degree oi correction. Correction for „ , ,, , , ,, , • 



, .■■,.■■ u- 1 . xu called a substage condenser and is nor- 



chromatic aberration is achieved at three „ . , . , . . ,. , 



,,,.,, J , , , mally equipped with an ins diaphragm to 



wavelengths m the red, green, and blue „ , , , , , 



,. , J 1 • 1 u X- • allow the observer to control the angular 



respectivelv, and spherical aberration is ,.,,•,,•• ^ i, n^, • , 



, , , , "^ J 1 , 1 xu 1 J cone 01 the illuminating bundle. 1 his control 



also held under closer control throughout , ,• , • • , 



,, . ., , , ,, . ., 1 . ^1 on the light permits one to attain the opti- 



the visible spectrum than is possible m the . , , 



, , . , , 1 „ mum compromise between contrast and re- 



achromats. Apochromats are also normally , . „, . . ■,, . r ^ 



, . , . T.y . ,, ,. solving power, ihis setting will be found to 



higher in JN.A. than corresponding powers ,-rr i i- i r- i 



. ,11, diner depending on the nature of the speci- 



in the achromats. r- o t- 



men. 



T^ . rr. Sometimes the light source will be external 



Eyepiece Ivpes , . , , , 



to the microscope, but the tendency is to- 

 Far and away the most popular eyepiece ward built-in sources, for obvious reasons of 

 for general microscopy is the Huygens eye- convenience and fool-proof operation. How- 

 piece. This comprises two separated simple ever, whether the source be built-in or ex- 

 plano-convex lenses, with field diaphragm ternal, the principle of Koehler illumination 

 located between the lenses. Despite its sim- is normally employed in this system, the 

 pUcity and inexpensive construction it is light source is imaged by a "lamp condenser" 

 well corrected for lateral color and works into the substage condenser. The latter im- 

 quite well with low and intermediate power ages the lamp condenser onto the specimen, 

 objectives. The virtue of this system is that it results in 

 For use with higher-power objectives, eye- a nice even field of illumination, even 

 pieces of more complex form are superior to though the source is of uneven brightness, 

 the Huygens eyepiece. These more complex as for example a coiled filament lamp. A 

 eyepieces compensate the lateral color aber- small amount of diffusion is sometimes added 

 rations of the higher power objectives, and to completely even out the illumination. Fig. 

 are particularly suitable for use with apo- 4 compares Koehler illumination with the 

 chromats. These eyepieces are sometimes formerly much-used critical illumination, 

 called "compensating" eyepieces, and in The most commonly used condenser is the 

 other instances are given specific trade two lens "Abbe condenser", having an N.A. 

 names. The various manufacturers recom- of 1.25 or 1.30 (see Fig. 5). The upper ele- 

 mend optimum combinations of eyepieces and ment is a plano-convex hemisphere, and the 

 objectives to achieve best correction, partic- lower element is bi-convex in shape. While 

 ularly in regard to lateral color. the aberrations of this system are sizeable, 

 For photomicrography special negative it nonetheless does a very creditable job of 

 (dispersive) lens systems are available, which illumination for any normal routine tasks, 

 markedly improve the correction of the im- It is sometimes made with the lower ele- 

 age for flatness of field. These negative sys- ment independently focusable as shown in 

 tems cannot be used as visual eyepieces, but Fig. 5 to provide better illumination of large 

 are strictly for photomicrography. fields in low-power work. 



449 



