422 SEC. 11. ASTRONOMY. 



at one end by a spring, while at the other end a string, passing over a pulley 

 and attached to a hook, can be made to hold the spring in a state of tension. 

 This done, on the thread being cut or burnt, the spring is allowed to act, the 

 shutter flashes across the image forming a cone- of rays, exposes the sensitized 

 plate, and the picture is produced. There are provisions for regulating the 

 exposure by an alteration of the width of* opening in the shutter, or by 

 increasing or diminishing the tension of the spring. 



The telescopic camera, attached to a bracket by means of two ring-clips or 

 couplings, accurately turned, providing a motion in arc for the camera tube, 

 read off by suitable scale and vernier, is bolted to the end of the declination 

 axis. 



Janssen's Apparatus. It is complete in itself and fits the same groove 

 provided for the reception of the ordinary 6-iuch collodion slide ; the position 

 planes of the sensitized plates also are identical in both, requiring no alter- 

 ation of focus- when using either description of slide. The apparatus is 

 supported upon a flat piece of wood, or " carriage," the lower portion fits 

 the camera groove, the upper part projects above the body of the camera 

 and affords the means of. attachment of a pinion wheel and arbor subse- 

 quently referred to. The carriage is perforated ; the opening admits of a 

 picture 7^" wide by I" high. 



The sensitized plate is held in position by a circular cell of wood sur- 

 rounded by a brass ring the periphery of which is racked with 300 teeth. 

 It is free to revolve between three friction rollers, a pinion wheel originally 

 cut with 12 teeth, of which 8 consecutive ones are removed, gears the racked 

 edge of the cell, and when turned uniformly by a winch (situated near the 

 centre of the telescope tube to prevent vibration) once per second during 

 fifty seconds or for fifty revolutions, the plate-holding cell will have revolved 

 once by an intermittent motion occupying one-third of a second and ceasing 

 or two-thirds. 



In front of the cell, i.e., between the sensitive plate and the carriage 

 opening, revolves a thin circular disc of brass, concentric with the plate but 

 of smaller diameter. It is racked on its edge with 240 teeth, and is caused to 

 revolve by a wheel having 60 teeth, which latter is mounted upon the pinion 

 wheel arbor, and is therefore moved by the same winch that turns the plate- 

 holding cell. The brass disc contains four apertures, equi-distant and other- 

 wise corresponding in shape, set to the one opening in the carriage. (To 

 regulate the exposure a mechanism is provided for simultaneously increasing 

 or diminishing all four openings.) 



The proportion of driver to follower being as 1:4 for one entire revolution 

 of the winch the disc turns through one fourth of its circumference, so that 

 when the Avinch is turned once in a second the cell containing the sensitized 

 cell moves on for one third of that time, the brass disc or shutter moving 

 also; after one third of motion the plate stops, but the shutter continues 

 moving, and one of its four openings flashes across that portion of the plate, 

 at that instant opposite the perforation in the carriage exposes and the 

 photograph is taken. This is repeated for 49 times in succession. Thus 

 49 photographic images of a limited space are taken in 49 seconds of time. 



Equatorial Mounting. This in outline resembles instruments constructed 

 on the German plan, but is " universal," i.e., admits of adjustment for any 

 latitude up to 80, either North or South of the Equator. A novel contrivance 

 has been introduced for retaining clock gearing for great variations of lati- 

 tude. Briefly the instrument combines all the most recent appliances for 

 convenience of manipulation, and, though massive as regards construction, it 

 may fairly be called portable. Eleven of these instruments have been sent to 

 various parts of the world, without, it is believed, one single mishap. 



