Maker's Name: 
Griffin & George Ltd
Where made: 
10 × 3.7 × 10 cm

A jolly little rough and ready illumination comparator invented  by Irish physicist, John Joly (1857-1933). Two blocks of translucent plastic sit side by side with a baffle between. One block is illuminated by a standard source at a fixed distance. The other block is illuminated by the unknown source. Illumination from a point source varies as 1/d2 where d is the distance from the source. So when the two blocks appear equally bright to an observer, the ratio of the source strengths is equal to the square of the ratio of the distances.