384 DR. J, C. MOTTRAM ON 



candles were used. In order to obtain a series of backgrounds 

 ranging from dark to light tone, the object was fixed on a 

 glass plate and was illumined separately from the background, 

 as shown in text-fig. 1, B ; by moving the background near to 

 or away from its illumination, and by using backgrounds of 

 different tones, it was possible to obtain every gi-ade of tone, 

 from black to white. 



In these experiments tone is alone considered ; colour was 

 kept constant by using only black, white, and neutral greys. 



(A) Plain Objects against Plain Backgrounds. 



Plain objects may enter into consideration of pattern, because 

 they are in reality objects covered by a very small pattern. 

 The visibility of plain objects was found to be afl:ected by the 

 following factors : — the human eye, the lighting, the atmosphere, 

 the background, and the object. 



The human eye, even when it does not present some gross 

 defect, nevertheless is found to vary from individual to indi- 

 vidual ; so that the readings made by one person cannot be 

 directly compared with those of others. Working with the 

 same eye, several factors afifect visibility ; if the eye be allowed 

 to become fatigued, the greatest distance at which objects can be 

 seen is much reduced. Experiments showed that from thirty to 

 forty observations could be made during two hours without 

 encountering fatigue effects. 



Some time must be allowed for the eye to become accommodated 

 to a sudden change in illumination : for a change from daylight 

 to almost complete darkness, twenty minutes is necessary; 

 working with two standard candles, it was found that fifteen 

 minutes must be allowed. 



The eye was also found to vary somewhat from day to day : 

 health and general fatigue are probably the cause of these 

 variations. Owing to this, one cannot directly compare the 

 reading of one day with that of another. The accuracy with 

 which measurement can be made is indicated in experiment 

 no. ]. 



The eflfect of the opacity of the air on visibility does not enter 

 into these experiments, as no measurements beyond eighty feet 

 were made, and observations were not made during fogs. 



The Effect of Lighting. — Experiments showed that the greater 

 the illumination the greater the distance at which objects can be 

 seen, all other factors remaining constant. 



The Effect of the Background.— A plain object is visible at a 

 great or small distance according as to whether the difference in 

 the amount of light coming from the object and the background 

 is great or small. A white object is more visible against a black 

 background than against a grey one, and more visible against 

 a dark-grey than against a light-grey one. Experiments were 

 not carried out to define this relation more accurately ; but the 



