Dr Hartridge, Colourimeter Design 277 



be about 29 degrees. Now, the dimensions of the field seen by the 

 eye are 8 mm. by 6 mm., the latter being in the direction of the 

 slope. The apparent diiference of focus is therefore less than 4 mm., 

 which would be equivalent to 12 mm. at a distance of 25 cms. 

 Such a small change of focus would be at once met by a trifling 

 change in the degree of accommodation of the eye, which would 

 be effected subconsciously and involuntarily. No difficulty is to be 

 met with therefore from this cause. 



The Mechanical System. 



The metal work of the colourimeter follows closely that of the 

 microscope. The horse-shoe foot, stage and coarse adjustment all 

 resemble those used in that instrument. The adjustment has a 

 range of 40 mm. only, because, as will be shown later, the use of 

 standard solutions of 20 mm. thickness makes a bigger movement 

 than this unnecessary. An accuracy of one-quarter per cent, should 

 be sufficient, and this is readily provided by a scale gi^aduated in 

 half mm. and reading by a vernier to one-twentieths. The adjust- 

 ment should have long, well-made V slides so as to eliminate lost 

 motion. The scale should be attached to the moving member, the 

 vernier being attached to the fixed. A simple lens and 45 degree 

 mirror should make a magnified image of this visible to the ob- 

 server. To the moving member is first screwed and afterwards 

 sweated with soft solder a strong brass ring. To this is attached 

 by means of a three-prong bayonet catch the ring fixed to the 

 upper lip of the movable trough. The trough is cemented into a 

 groove turned in this ring by means of plaster of Paris or Caemen- 

 tium. Where plaster has been used the joint should be covered 

 by a thin coat of Robiallac. The prisms and eyepiece are attached 

 to a strong projection at the top of the pillar which forms the 

 handle of the instrument. 



The removal of the troughs for filling and cleaning and their 

 replacement is a simple process which should not take more than 

 a few seconds. To remove the adjustable troughs, first swing the 

 substage to one side ; this allows the lower trough to drop verti- 

 cally through the hole in the stage until it can be removed. The 

 upper trough is now gripped between the finger and thumb, and 

 the trough rotated so as to free the bayonet catches ; this trough 

 is then lowered through the hole in the stage and removed. The 

 plunger and the troughs can now be cleaned, refilled and returned. 

 The standard double trough simply rests on its side of the stage, 

 so that its removal takes but a moment. 



