VISION 127 



21. Lantern test. The Edridge Green lantern test is 

 more practical than the above, for with it the conditions 

 can be closely imitated under which railway and light- 

 house signals have to be correctly recognised. The lan- 

 tern consists of a circular box which contains three ro- 

 tatable discs. These discs are drilled with holes which 

 are then fitted with glass windows. The colours in the 

 first and second discs are, standard signal red 1 and 2, 

 yellow, green, standard signal blue-green, blue and 

 purple. The third disc contains ground glass to repre- 

 sent mist, ribbed glass to represent rain, four neutral 

 glasses to represent fog of different density. 



Behind the lantern is fixed an electric lamp, the light 

 from which passes through an aperture in the box and 

 so through the glass windows in the discs when they 

 are rotated into position. 



In front the lantern has an iris diaphragm by which 

 the size of the coloured lamp image and therefore its 

 apparent distance can be varied. 



The lantern is placed on a table in the dark room and 

 is connected to the electric light supply. 



A sits opposite the lantern and as far from it as pos- 

 sible. B switches on the light and asks A to describe 

 what he sees. By rotating the discs, B interposes suc- 

 cessively the coloured glasses alone and with the other 

 glasses between the light and A, and A in each case 

 describes what colour he sees. The normal sighted will 

 recognise the red, green, yellow or white lights, whether 

 modified or not. A common mistake of the colour-blind 

 is to call red or green, the yellow glass when combined 

 with one of the neutral tint glasses. 



