266 ANNUAL REPORT SMITHSONIAN INSTITUTION, 195 7 



motion on the main part of the machine. In nearly all cases the 

 lamps are tungsten-filament electric, and are part of a projection 

 system that includes a condensing or light-gathering unit, a diaposi- 

 tive or its equivalent, and an objective or projection lens system 

 which focuses on the dome an image of the illuminated diapositive. 

 The diapositives for the star-field projectors are not photoemulsions 

 on glass, but pieces of copper foil with small round holes punched 

 in them for the stars. These punchings are varied in size in accord- 

 ance with the brightness of the real stars they represent. Holes 

 for the faintest stars are of the order of one-thousandth of an 

 inch in diameter. The 16 star-field projectors mounted in each ball 

 at the ends of the dumbbell are lighted by the one light in the center. 



A further point of interest regarding the projectors is the pro- 

 vision made in all of them to cut off their light when they are 

 pointed below the horizon, thus keeping their direct light from the 

 eyes of the audience. In nearly all instances the occulting device 

 is a cup-shaped, gravity-operated shield that slowly swings into 

 the projection beam as the projector is tilted downward. 



The prime movers for the machine are small 3-phase alternating- 

 current motors; reversal of phase accomplishes reversal of direction 

 of rotation. They are all mounted on the main moving part. 

 Transmission and interconnection are accomplished by gearing. 

 Motions that are additive are joined through planetary transmissions. 

 The motions and lamp circuits are all controlled remotely by 

 the lecturer from a switchboard in a speaker's stand near the 

 wall of the room. Here on the horizontal part of the main 

 board are labeled switches for every motor or lamp, and rheostats 

 or powerstats for controlling the brightness of the lamps in use. 



It might be helpful at this point to differentiate between the 

 planetarium as a device or training aid, as just described, and the 

 planetarium as an institution, its more appropriate usage in the con- 

 text of current-day function. All the planetariums in the United 

 States are organizations that serve several purposes, though the 

 popular program of explication in astronomy is usually the primary 

 mission. No matter whether the projection instrument is used or not, 

 one refers to the "planetarium" when speaking of the organization. 



ADLER PLANETARIUM 



Aside from the orreries, globes, and armillary spheres that made 

 their appearances in many schools and museums, the first planetarium 

 venture in the United States was brought to reality in Chicago. Max 

 Adler, a former official of Sears, Roebuck & Co., generously donated 

 $500,000 to the city of Chicago to purchase a Zeiss planetarium instru- 



