370 Capt. Abney and Major-General Festing. [Jan. 28 r 



Extinction of the Light of different Parts of the Spectrum. A full 

 description is given of the apparatus used and of the method of 

 observation for determining how much the light of each part of the 

 spectrum must be reduced in order that it may be extinguished. 

 Observations were made (1) with the central part of the eye only 

 and (2) with the whole eye. 



From these observations curves were plotted, showing the propor- 

 tion of the beam from each part of the spectrum which was just not 

 visible. These are called extinction curves. They differ only in that 

 part of the spectrum where the yellow spot absorption takes place. 

 The minimum ordinate is at about X 5300, and represents 65/10 7 , that 

 being the proportion to which the beam had been reduced at extinc- 

 tion, the intensity of the unreduced beam from D in the same 

 spectrum being that of an amyl acetate lamp at 6 feet. The intensity 

 of other beams of the spectrum was calculated from this by the aid 

 of the luminosity curve as before. A curve was then derived from 

 each of the extinction curves by taking as ordinates the product of 

 such ordinate of an extinction curve and the luminosity of the corre- 

 sponding beam; these derived curves then represent (on the supposition 

 that all the beams were originally of the same luminosity as D), the 

 proportion, and therefore the absolute intensity, of any beam which 

 would be just not visible. These two curves differ slightly at the 

 part affected by the yellow spot, but that for the whole eye is hori- 

 zontal from the extreme violet end to about X 4800 ; it then rises rapidly 

 to A, 6840, and again becomes horizontal. This seems to confirm the 

 view that a single sensation only is excited by each of the ends of the 

 spectrum. 



The reciprocals of the ordinates of either of the first two extinc- 

 tion curves being taken, what is called a " persistency " curve is 

 formed. The curve for the whole eye and that for the central 

 portion are given. It is reasonable to expect that the " persistency " 

 curve should have relation to some colour sensation of the eye, which 

 may perhaps be looked upon as the dominant sensation, as it is ex- 

 cited by the smallest quantities of light. 



An examination with the results of observations made by colour- 

 blind people is then entered on. 



A gentleman, M., made a series of observations. He has two colour 

 sensations only, which he calls "red" and "black." Yellow he 

 describes as " white," green as " bright black," blue as " darker 

 black." His luminosity curve has been plotted on such a scale that 

 the red portion corresponds with that of the normal curve. The rest, 

 however, falls below this, and it leaves off a little beyond F. The 

 curve formed by the differences of his and the normal ordinates may 

 be considered to be M.'s deficiency curve. 



Two brothers, P. and Q., were also examined. They have the same 



