212 Botanical Microtechnique 



Fig. 17.7— Horizontal apparatus for use with Micro Tessar objectives. Components, 

 from left to right: ribbon filament lamp; filter holder; revolving stage with con- 

 denser holder; bellows camera with removable lens board, which carries the focusing 

 mount and a behind-the-lens shutter. The commercial focusing screen spring-back 



is renio\al)le. 



of control. A 6-volt, 108-watt coil filament or ribbon-filament lamp 

 furnishes a steady, fixed source of adequate intensity. A transformer 

 furnishes G-voh current from the llO-volt alternating-current line. 

 A rheostat may be used to control the intensity if color temperature 

 is not critical. The tungsten-arc and zirconium arc also are excellent 

 illuminants. The carbon arc has a brilliant, homc:)geneous crater, 

 but the crater shifts as the carbons bmii away, and it is difficult to 

 keep the crater exactly in the optical axis. 



The lamp must be provided with an adjustable condenser and 

 an iris diaphragm. A one-lens spherical condenser or the slightly more 

 expensive aspheric condenser will give good results, biu a better 

 corrected condenser with two or more components is preferred. 



Focusing Aids 



1 he focusing jjanel in commercial apparatus is usually mack' ^vill; 

 sufficient precision to place the ground glass in the same plane as 

 ilif |)hoiographic enmlsion. If correct focusing is not obtained, the 

 iua(( urate positioning of groiuid glass is suspected, the easiest remedy 

 is ilie use of a |)Iate holder as a focusing panel. Rtinoxt' the partition 

 that separates ilu plates in a doubli' iioldci. insert a ground glass 

 into I he plate grooves. This j)la(cs the ground stirface in the same 

 jjlane as the emulsion. Take the photographs with a plate or film 

 holder of the same make as the one used as a focusing panel. 



The groimd-glass surface prox iiles a satisfactory image for orient- 



