72 Prof. F. Allen on Four Transition Points in the 



parts to the arbitrary value of 100, and the values of the 

 abnormal curves in the same ratio. Slight departures of 

 the abnormal from the normal curves have been ignored, 



Fiff. 16. 



long experience having shown that they are without signi- 

 ficance, and to be attributed rather to the inevitable errors of 

 observation. 



The elevations of the curves, both single and double, can 

 be readily recognized and compared, and their limiting wave- 

 lengths noted. As they are arranged in ascending order of 

 wave-lengths, with the shortest, violet, at the bottom, the 

 curves for the four transition points fall into their proper 

 places, and show their relation to the curves of the other 

 types. Each band of colour used for fatiguing the eye is 

 indicated by a short vertical line just above the normal 

 curve. 



Finally, the seventeen curves have been consolidated into 

 a single composite curve by taking the mean values for all 

 the elevations of all the curves. These values are shown in 



