PROCEEDINGS. 303 



The instrument is intended to analyse the colour of any substance 

 either in solution or solid, by the use of coloured glasses accurately 

 graded, into two of the primary colours red, yellow and blue with 

 so much shade added. In his early trials the great difficulty was 

 to get a colourless glass as a basis. Mr. L. first started with 

 dark tints of the above three colours and took as his standard the 

 least difference that could be seen between two glasses of the same 

 colour which was called 1 for each ; for lighter shades in which 

 the perception of variation in colour is more readily noticed, this 

 standard was divided into tenths and hundredths, Each of the 

 three colours was so far treated independently until it was found 

 that the mixture of these standards in the proportion of 1 of red 

 1-2 of yellow and 2*4 of blue produced a neutral grey at all depths 

 of shade, until, with the deepest, blackness was the result. The 

 standard was therefore altered so that 1 red, 1 yellow and 1 blue 

 produced neutral grey, which made it possible to register any tint 

 in terms of two colours plus so much grey. Thus in three of the 

 coloured papers which I have to show you, the first a greenish- 

 blue registers 2*5° yellow and 4-4° of blue simply — the second a 

 dark crimson colour registers 16'5° red, 1*2° yellow, and 1° blue, 

 and as 1 blue, 1 yellow and 1 red produces 3 of grey, the corrected 

 reading is red 15*5° yellow 0*2° and grey 3°. In the third, which 

 is pale yellowish-brown the tests show 2-5° yellow, 2*5° red, and 

 1*1° blue, so the corrected register becomes 1-4° yellow, 1*4° red, 

 and 3*3° of grey. 



The instrument itself consists of a tapering tube divided in two 

 by a partition with an observing opening at one end, so that on 

 looking through two oblong fields are seen together. Opposite 

 one of these fields the object to be observed is placed and into 

 slits in the tube slides of coloured glass are placed until equality 

 is produced. In the case of fluids, cells from J- inch to 2 inches 

 or more in depth are used ; these have parallel sides as that the 

 image is pure, and the light is obtained by reflection from a sur- 

 face of opal glass ; in diffuse daylight with opaque objects a tray 

 of plaster of Paris is used as a reflector behind the coloured glasses. 



