274 Dr Hartridge, Colourimeter Design 



however, that reflection can still take place at the sides of the 

 troughs, so that it is necessary carefully to restrict the light illu- 

 minating the troughs to narrow vertical pencils of just sufficient 

 diameter fully to illuminate the comparison fields. Since scattered 

 or reflected light may increase the apparent brightness of one of 

 the fields it is essential that this be reduced to a minimum. Special 

 care should therefore be taken in designing the instrument to pre- 

 vent the entrance of stray light, and to employ an illuminating 

 system that will limit the entering beams to the narrow pencils 

 above referred to. 



The illumination in the majority of colourimeters is obtained 

 from the sky by means of a plane mirror. In some instruments this 

 may be replaced at will by a finely matted white surface. The 

 illumination therefore in either case consists of a large number of 

 divergent pencils, which enter the lower ends of the troughs in all 

 possible directions. Scattered light is therefore at a maximum. In 

 the case of the microscope a similar practice used to be in vogue, but 

 it has given way to the use of illuminating lens systems in which 

 the corrections and alignment are well nigh as perfect as those 

 used in the objective and eyepiece. Now, in the case of the spec- 

 trophotometer I have shown that the beams illuminating the two 

 limbs of the instrument should proceed from identical parts of the 

 light source. This condition should be realised in the case of the 

 colourimeter also. The arrangement of the illuminating apparatus 

 is shown in the diagram. 



The light source is similar to that which I have applied to the 

 microscope (2), consisting of a slab of white opal glass finely 

 ground on both sides. This is lit from behind by means of a small 

 half watt electric lamp, which obtains its current from a small 

 accumulator or dry cell, or from the town supply through a suit- 

 able resistance. The lamp is enclosed in a brass box, which is 

 silver plated inside, and is finished dead-black outside so as to 

 radiate heat. The life of the lamp is increased by connecting it 

 with a press switch so that it is in circuit during observation only. 

 The lamp box is attached to the tail-piece of the instrument so 

 that it forms an integral part of the apparatus. The whole may 

 thus be tilted or moved from place to place without requiring re- 

 adjustment. Immediately above the opal glass is a metal dia- 

 phragm, the aperture in which limits the surface exposed to a 

 disc 4 mm. in diameter. Attached beneath the stage of the in- 

 strument and 60 mm. above the diaphragm of the light source is 

 a plano-convex achromatic lens of 26 mm. diameter and 60 mm. 

 focal length. The divergent rays from each point of the source 

 are rendered parallel by this lens, and at once pass through two 

 achromatic plano-convex lenses of 18 cms. focal length and 14 mm. 

 diameter. These lenses have a clear aperture of 12 mm. and form 



