Studies in Visual Sensation. 



461 



mental work obtain a series of rings which do afford approximately 

 equal steps from black to white — of which any intervening ring 

 appears to be of an intensity or shade which is the arithmetical mean 

 between its neighbours on either side. This may be done by means of 

 slit discs on Maxwell's method, giving sectors which slide over each 

 other so as to alter the relative proportions of the white and black. 

 First a mid-grey may be found, which appears to give a half-way sen- 

 sation between black and white ; then other greys, which appear to be 

 arithmetical means between the mid-grey and black on the one hand, 

 and on the other hand between the mid-grey and white. Thus by a 

 series of careful adjustments rings may be obtained which enable the 

 eye to pass from black to white by steps which are of approximately 

 equal value for sensation. 



It is not, however, easy to judge of the exact equality of the sensa- 

 tion increments. It is not easy, for example, to say what shade of 

 grey stands just midway between black and white; and with four 

 steps, even when one judges them to be approximately equal, one feels 

 that there is equality with a subtle difference. The step from black to 

 dark grey may be substantially similar in value to that from light grey 

 to white ; but it is not the same ; and there is the disturbing element 

 of contrast causing the rings to lack uniformity of shade. One feels 

 that the method of rings giving equal sensation increments can only 

 give a first approximation to a scale of sensation. For what they are 

 worth, however, let us consider the results. 



Admitting that we have reached a first approximation towards an 

 evenly graded series of sensations, we have at least advanced a stage 

 towards the establishment of an arbitrary unit of sensation. We have 

 obtained a scale or ladder from black to white. How shall we deal 

 with it % Let us term our black the zero of an arbitrary scale, and our 

 white 100 per cent. We must realise, however, that our zero, which 

 we term black, is simply a datum level from which to reckon. That 

 which I employ is a dull black surface paper coated with black enamel. 

 This gives a bright reflecting surface ; but it is not difficult so to 

 arrange matters that the scanty light reflected to the eye from its sur- 

 face is derived from black velvet' or cloth hung in a dark corner. Still 

 it is not, and it makes no pretence to be, absolute black. Let us 

 assume that it is a very dark grey, and let that be our zero of stimulus 

 and also our zero of sensation. So too at the other end of the scale. 

 Our white paper affords an arbitrarily selected luminosity under given 

 conditions of illumination, and we call it 100 per cent, of stimulus, 

 corresponding to 100 per cent, of sensation. We have thus a per- 

 centage scale — I repeat again a purely arbitrary percentage scale — for 

 both stimulus and sensation, by means of which we can bring them 

 into relation to each other within the assigned limits. 



Let us now compare the results we have so far obtained, stating 



