286 MICRO-PROJECTION WITH SUNLIGHT [Cn. IX 



better source of light for the microscope than the lime light, and 

 now electric lighting is so common that one can use almost any 

 room in a house or laboratory at night for a projection room. 



Of course one should not expect too much, but for small audiences 

 50 to 100 and with a moderate sized screen 2-3 meters 

 (6-10 ft.) astonishingly satisfactory micro-projection can be done. 



418. Hand-feed and automatic lamps for small currents. 



Most of the small current lamps are of the hand-feed type whatever 

 the form of the electric current (a. c. or d. c.) but some automatic 

 ones have been constructed (fig. 44, 205). Large arc lamps may, 

 by special arrangement, be so adjusted that they give good results 

 automatically from 5 to 25 amperes (e. g. the automatic lamp of 

 A. T. Thompson and of the Bausch and Lomb Optical Co., fig. 

 186, 187). 



As for the usual lantern arc lamps, only those for the direct 

 current have hitherto been constructed of the automatic form. 



For a full discussion of the wiring and setting up of the apparatus 

 see Ch. Ill and XIII, 128 and fig. 3, 40, 45. 



Do not forget that a rheostat or ballast of some kind must be 

 used on every outfit where an arc lamp is employed ( 129, 748). 



Remember the precautions for turning on and off the current 

 when using the house circuit ( 133). For a further use of these 

 small currents in drawing, see Ch. X, 486. 



MICRO-PROJECTION WITH SUNLIGHT 



419. This was the first light used for micro-projection and 

 remains the best. If it were only available at all times it would 

 be universally employed. 



420. Arrangement of the parts of the apparatus. For the 



heliostat to keep the sunlight in a constant position one should 

 consult Chapter VI. 



After getting parallel light from the sun in a constant position, 

 then one should use the proper condenser (fig. 74). The remainder 

 of the apparatus is precisely as for the projection so far discussed 

 and all the requirements of centering and arranging at the proper 



