54 Froceeclings of the Royal Societij of Victoria. 



central electro magnet, keeping tbe lever neuti-al. Now, if 

 the driving clock moves the telescope accurately, the 

 astronomical clock current arrives at the moment the 

 detector is making contact with the central spring No. 2, 

 and the distributor remains neutral ; if, however, the 

 driving clock goes a little too fast, the current passes 

 through No. 1 spring, and the distributor makes contact 

 with the lei't platinum point, and if if goes too slow, the 

 current passes through No. 3 spring and moves the 

 distributor to the right platinum point. This works so well, 

 that if the telescope moves ~ of a second too fast, or too 

 slow, it is innnediately detected, and works the distributor. 

 Now tbe "distributor" distributes another battery current to 

 a little mechanism called an accdevator and retarder, whose 

 offices are respectively to accelerate or retard the driving- 

 clock by veiy small amounts, according to the operation of 

 the distributor, which is instructed, if one may use the 

 word, by the detector. By this means the telescope can be 

 kept following the stars so accurate!}' that any one bisected 

 by the micrometer spider web, will remain there for an hour 

 or more together. The accelerator and retarder apparatus 

 is composed of a pair of accurately constructed epicyclical 

 trains, which cannot be described without models or 

 complicated drawings. Besides these regulators, which 

 are automatical!}' worked as described, there is ai«.other 

 pair workable by a hand key with two studs, and battery, 

 so that by pressing one stud it operates the retai'der and 

 the otlier the accelerator. 



The following will give a fair idea of the actuid work of 

 photographic chai-ting : — The paiticular parts of the zones 

 to be photographed on a night are arranged beforehand, 

 and the guide stars for each plate selected. The dome being- 

 opened up and driving- clock set going, the observer sets tbe 

 instrument on the guide star, and as soon as the telescope is 

 found to be steadily following, an assistant brings the plate 

 holder armed with a photogra})hic plate and inserts it into 

 the plate holder fi-ame of the telescope. The time of 

 exposure being previously settled upon, the observer as soon 

 as all goes quite steadily, opens the exposing shutter, 

 keeping his e}-e constantly on the guide star, which is now 

 bisected at the cross of the spider webs. The tiuie oi" 

 opening the shutter is noted, and at the expiration of the 

 fixed time of exposure, as shown by an astronomical clock 

 in the observing room, the assistant warns the observer, 



