1886.] 



Colour Photometry. 



239 



of the two shadows can be distinguished with great exactness. The 

 authors describe their method of fixing the position of the rays employed 

 and the source of light with which the spectrum is formed. They also 

 enter into details as to the comparison light, the receiving screen, 

 and the comparative value of the light as seen by them respectively. 

 The curve of the intensity of the arc light spectrum, as seen by their 

 eyes, which they call the normal curve, is then described. The 

 question as to the effect of an alteration of the colour of the com- 

 parison light is then discussed, as is the effect of the brightness of the 

 spectrum. 



The next point touched upon is as to the value of mixed light as 

 compared with its components. It is found that the following law 

 holds good, viz. : that " the sum of the intensities of two or more colours 

 is equal to the intensity of the same rays when mixed." This law is 

 applied to Hering's theory of colour. 



The authors next state that with the majority of people the curve 

 of luminosity of the spectrum is identical with the normal curve, but 

 that in some cases slight differences may be observed, of which one 

 example is given. Such slight deficiency does not constitute colour- 

 blindness, since the want of appreciation of any colour is but very 

 partial. They next describe observations made by four colour-blind 

 persons, and show that there is a remarkable divergence in their 

 curves from the normal. The deficiency curves are shown, from 

 which it appears that two of the observers are totally blind to red, 

 whilst the other two are partially so. They then show that such 

 observers would not give a true value for any light which is not of 

 identically the same colour as the comparison light they might 

 employ. It also appears that the intensity of illumination felt by 

 a colour-blind is really less than that perceived by a normal-eyed 

 person. 



Two examples of the normal curve for sunlight are then given, one 

 taken on a day in July by the method of separating close lines by 

 means of varying illumination, and the other in November, by the 

 method adopted by the authors. Their results are compared with 

 Vierordt's curve, obtained by extinguishing colour with white light. 



In order to ascertain the effect of the turbidity of a medium through 

 which light passes (for instance sunlight), the authors compared the 

 intensity of the spectrum after passing through clear water and turbid 

 water, and found that the absorption agreed with Lord Rayleigh's 

 theoretical deductions that I' = I e~** A ~*, where I' is the intensity after 

 passing through a turbid medium, I the intensity after passing 

 through clear water, x the thickness of the turbid layer, k a constant 

 independent of A,, \ being the wave length. 



The authors conclude their paper with a discussion of the intensity 

 of incandescence of carbon electrically heated. 



