€2 



Prof. F. Allen on Four Transition Points in the 



had not been sufficiently fatigued or that the readings were 

 not properly taken. With the greatest care the whole of the 

 readings were repeated on the following day with exactly the 

 same result. The remarkable conclusion follows, that fatiguing 

 the eye with a narrow band of colour whose central wave- 

 length is \*660 ft, has not the slightest perceptible effect on 



Fio-. 6. 



•045 



040 



•035 



030 



51 



025 



•020 



•015 



Eye fatigued with 



A -625{A 



Norma/ — 



•40// "45 -50 -55 -60 



Wave Length. 



•65 70 



•75 



the measurement of the persistence of vision for any part of 

 the spectrum. This wave-length, X '660 p, is therefore the 

 transition poi\\t between curves of the single red elevation 

 type and those oLthe double red and green elevation type. 



