274 Transactions of the Society. 



merely comparative, and tliat the controlling factor (or more 

 correctly, one of tbe controlling factors) is that of contrast be- 

 tween the successive luminosities. In any of these examples the 

 eye takes an appreciable time before it has recovered from the 

 " shock " imposed on it by the comparatively bright light and 

 can see again at its best in the comparatively dark light. 



It is obvious that it is not the brilliancy of the light itself 

 which is the main factor to cause the strain and consequent lack 

 of accuracy in seeing, for under normal conditions no fatigue is 

 caused by prolonged observations in daylight, for example, or with 

 good artificial illuminants. The eye accommodates itself to a 

 very considerablc"range of luminosities, and, unless the luminosity 

 is rapidly changed, works easily, with accuracy and without strain. 



Endless instances might be cited to show how apt we are to 

 judge of particular degrees of luminosity simply by reference to 

 others, and how the same luminosity affects or does not affect the 

 eye in consequence. If, for example, the artificial light in a room 

 is turned on in the day the eye suffers no inconvenience ; one 

 can look at it not only without producing so-called blindness, but 

 Avith a feeling of astonishment, arising from the impression that 

 the voltage or gas pressure should be so bad that such a poor light 

 is obtained. 



Or consider the opposite case, when a photographer with his 

 eyes adjusted to daylight or briglit artificial light goes into the 

 comparatively feeble light of his dark-room. Suppose he wishes 

 to develop panchromatic plates, and is using an ordinary Wratten 

 and Wainwright green safelight. Many can testify that upon 

 entering the dark-room they cannot detect whether the gi'een light 

 is on or off, whilst after twenty to thirty minutes not only can they 

 distinguish the objects in the surrounding room, but the light 

 appears so bright that they begin to wonder whether it is possible 

 for so bright a light to be safe with panchromatic plates. 



The examples cited suffice to show that whether ^ve move 

 from a comparatively dark luminosity to a comparatively bright 

 one, or vice versa, the adjustment of the eye to its new surround- 

 ings is a slow process ; it might even be said to be a very slow 

 process, since the eye seems to improve in seeing-power for one 

 or two hours after going into a dim light, although the improve- 

 ment gradually falls off and becomes less and less marked after 

 the first fifteen to twenty minutes. 



For want of a better expression, I have used the term " shock " 

 to designate the effect on the eyes of sudden changes from dark to 

 bright luminosities, or vice versa, but there is also the further 

 effect, which 1 shall refer to as " strain," arising from the conscious 

 or unconscious effort of the eye to accommodate itself to the new 

 degree of luminosity, and to see clearly, before in fact it has had 

 time to adjust itself properly to the new circumstances. 



