140 Scientific Proceedings, Royal Dublin Society. 



casting quite independent of the transit instrument, but so placed 

 that a prolongation of that polar axis would pass through the 

 horizontal axis of the transit and the optical centre of the object- 

 glass, and the metal arc would be so constructed that its centre 

 also coincides with the same centre of the transit instrument. 



With this construction, the photo-plate, whose carrier can be 

 slid up and down the arc, will always be at the focal distance of 

 the object-glass from the optical centre of that glass; and therefore 

 any star-image falling on it will be in correct focus ; and if the 

 polar axis be driven uniformly by clockwork as in the ordinary 

 equatorial, the plate will always travel at the correct rate for 

 images of any declination formed upon it ; consequently a plate 

 exposed in this apparatus should, when developed, show a round 

 image of a star. 



For registering the seconds on the same plate, a diaphragm 

 would be mounted at one end of the axis of the transit instrument 

 with optical means by which an image of that diaphragm will be 

 formed on the same plane as the star-images. This diaphragm 

 would be strongly illuminated by a small electric lamp, and sup- 

 plied with some kind of instantaneous shutter so arranged that 

 every second, or every alternate second, an image of this diaphragm 

 would be flashed on the plate. 



Preferably the design cut on the diaphragm would be as in 

 fig. 2, so that the plate when developed would be as in fig. 3. 



Fig. 2. Fig. 3. 



It will be observed that every fifth second is " dropped " — that 

 is to say, every fifth flash is omitted on one side, while every tenth 

 second is dropped on the other side. This is to facilitate the 

 identification of the seconds. 



