100 Scientific Proceedings, Royal Dublin Society. 



3. All differences in the angle of reflection of light by the original are 

 avoided, all tlie effective light emerging at riglit angles to the surface. 



This last circumstance places at our disposal a delicate and accurate 

 method of comparing the reflecting powers of surfaces for vertical incidence. 

 It consists in mounting the surfaces in the same plane side by side, and 

 exposing a plate with its film in contact with botii. If the surfaces are large 

 enough, a slight gap may be left between the sensitive film and the surfaces 

 to be tested, so as to eliminate photographically active radiations or 

 emanations from the surfaces. 



