ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



99 



illumination, bus a movement of about 4 mm. towards the lens-openincr 

 so that the incandescent crater of the upper carbon is formed towards 

 tbe _ front. _ For spectral work the lamp is set upri^•ht and is therefore 

 eqmpped with revolver carbon-holders which facilitate a quick readjust- 

 ment of carbons when required. The automatic regulatinj? lamps for 

 constant current (fig. 12) and for variable current (fig. 13 V are 

 constructed on the differential principle and regulated for an assigned 

 current-strength. In the choice of current-strength attention must be 

 paid to the desired magnification as well as to the size of the audience 

 room. In the use of good transparent diapositives of H by 10 cm 



Pig. 18. 



and of an image-size of 2-3 metres square at a projection distance of 

 5-8 metres a constant-current lamp usually requires a current-strength 

 of 10-12 amperes ; 8-10 metres projection distance requires 1,5-20 

 amperes ; and greater distances 20-;-50 amperes. For a change-cuiTent 

 double these strengths should be taken. To attain the most favourable 

 light-values for projections with automatic constant-cuiTent lamps, the 

 inclined position should be adopted and the carbons regulated as with 

 the hand-lamp. For spectral work the perpendicular arrangement is 

 required. The variable-current lamp is always set perpendicularly, the 

 lower carbon, in projection work, being slightly advanced. A trans- 

 tormer of 50 volts secondary range is supplied with the variable-current 

 lamp. Figs 14-17 show the lanterns for projection with the two kinds 

 of current ; fig. 14 showing the lantern open with hand-regulating lamp, 

 ngs. 15-17 an automatic-regulating lamp. The projection apparatus, 

 with complete optical equipment consisting of optical bench, leather 



H 2 



