Photometry of Lights of Different Colours. 863 



indicated that observer 10 should measure the tungsten 

 lamp against the carbon as 3*3 per cent, higher than observer 

 14. An actual test, where the two lamps were measured 

 against a comparison lamp of intermediate colour, gave a 

 difference of 2'8 per cent. By this method of correction, 

 therefore, a difference of nearly 3 per cent, in the readings 

 of the two observers is reduced to ^ per cent. As these 

 differences are of the order of magnitude of the errors with 

 the planimeter and photometric methods, the test is taken 

 to show the validity and feasibility of such a method of 

 correction. 



A possible and very attractive means of incorporating the 

 average eye into photometric measurement is the develop- 

 ment of some sensitive radiometer to supplant the eye. Such 

 an instrument so screened as to respond at the different 

 wave-lengths in accordance with the data of Table II. would, 

 if sufficiently sensitive, offer the most acceptable solution to 

 the problem. Perhaps the photoelectric cell will do this. 



Summary. 



This paper brings to a close the investigation on coloured 

 light photometry as previously laid out. The practical 

 result of the study has been to arrive at a choice of instru- 

 ment and of conditions best suited to this kind of photometry, 

 and to establish the characteristics of the average eye by 

 which such measurements are to be carried out. By con- 

 fining the actual comparison of differently coloured lights to 

 the standardizing laboratory, so that all practical photometry 

 is that of lights of the same colour, a solution of the problem 

 is offered through the use of the results of this investigation. 

 The results of the photometric method here advocated hold 

 for the average eye under the most important illumination 

 conditions. It is inevitable that correction factors will need 

 to be applied to these values whenever the effective illumina- 

 tions are widely different from that here adopted as standard. 

 But such corrections will apply to the less frequent and less 

 important conditions, that is, to ones where the loss in 

 accuracy involved by the use of more or less rough correc- 

 tions is of least importance. 



Physical Laboratory, 

 National Electric Lamp Association, 

 Cleveland, Ohio. 

 August 1912. 



3 L 2 



