PROCEEDINGS 



OF THE 



Colourimeter Design. By H. Hartridge, M.D., Fellow of 

 King's College, Cambridge. 



[Received 7 October 1919; read 10 November 1919.] 



In a previous paper (1) I have described certain factors which 

 affect the efficiency of the spectrophotometer. The colourimeter 

 has been found to be similarly affected, so that various modifica- 

 tions in the usual designs are indicated. 



The comparison field is in most instruments divided at a 

 diameter, so that one half receives light which has passed through 

 one limb, and the other half light that has passed through the other 

 limb of the instrument. In a few designs the bull's-eye and the 

 central strip fields have been employed. All these fields have 

 the disadvantage that local stimulation of the retina may occur that 

 sets up after image phenomena greater in degree in one part than 

 in another, thus preventing accurate determinations. And, further, 

 they do not make the best use of the effects of simultaneous contrast. 

 A better type of field is the one which I have previously described 

 in connection with the spectrophotometer, namely, one which is 

 subdivided into a number of strips, of which alternate numbers 

 receive light from the two limbs of the instrument. With this field 

 the eye does not select any one part for examination, but tends rather 

 to judge of the field as a whole. When the adjustment of intensity 

 has been correctly made the whole field should become uniform. 

 The effects of retinal fatigue therefore tend to become uniformly 

 distributed. The contour of this type of field is of considerable 

 length compared with its total area ; the conditions are therefore 

 beneficial for the development of contrast. The absence of visible 

 lines of junction still further increases this effect. 



The prisms A and B by which the beams of light through the 

 two limbs of the instrument are combined at the compound field 



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