ARRANGEMENT OF APPARATUS. 139 



It is sometimes difficult to tell if the srm illu- 

 minates the object equally when a heliostat is not 

 used. The most convenient method of ascertaining 

 this, is by observing the image formed on the sensi- 

 tised plate through a piece of ruby glass let into the 

 shutter of the plate carrier, or in the top of the 

 camera itself. The former is the better method with 

 dry plates, the latter with wet. 



