PHOTOGRAPHIC SURVEYING. 127 
better finished as theodolites have been made by Troughton and Simms. 
For example, their instruments carry a larger telescope which revolves 
on its axis, so that it can be used for sights fore and aft, and it is 
reversible in the Ys; there is also a complete vertical limb, divided on 
Fig. 2. 
silver, and a level on the telescope. The horizontal limb is also divided 
on silver, and there are two verniers at opposite ends of a diameter, and 
various other details which render the instrument superior as a theodolite 
to that made by Casella. Trougliton and Simms charge £50 for their 
instruments. As surveying, cameras the instruments are practically 
