398 

NATURE 
[JUNE 10, 1915 

is very great, and secondly, because the object 
viewed is below the resolving power of the eye 
(about one minute of arc), and therefore the 
dimensions of the object viewed have not to be 
considered. The visibility of such point sources 
of light is inversely proportional to the square of 
the distance of the observer from them, and 
directly proportional to their candle-powers. Their 
visibility becomes an important matter at sea, 
since the navigation of ships so as to avoid 
collision depends almost entirely on them. A 
practical working visibility is 1°76 candles at two 
sea miles, whether the light be red, green, or 
yellow. There is a very marked difference, how- 
ever, in the relative behaviour of these lights to 
oblique vision. To normal people a green light 
will appear some five times brighter when viewed 
obliquely than when viewed directly, but a red 
light acts in exactly the opposite way. If we place 
a point source of green light in a dark room so 
that its visibility from the position viewed is equi- 
valent to about o’o1 micro-candle at a metre, most 
observers will pick up this light when looking 
in some other direction and be quite conscious of 
its presence, but no sooner does one look towards 
it than it is gone. A faint red light, on the other 
hand, cannot be picked up by oblique vision at all, 
but when an observer has finally discovered it by 
direct vision he is impressed by how very visible 
it is. If he glances to one side, however, it dis- 
appears. The colour of faint lights is not dis- 
tinguishable by oblique vision. 
Under favourable laboratory conditions of 
threshold visibility it is calculated that the energy 
which is just sufficient to stimulate a single element 
of the retina of the dark adapted eye is at the 
rate of about 10~! watt. 
The question of the visibility at night of objects 
of finite size is not so simple. The problem 
occurs in a practical form in the showing-up of 
an obstacle by a motor headlight, such, for 
instance, as a pedestrian on the road ahead of a 
car. Such objects subtend angles well above that 
of the resolving power of the eye, and there is no 
justification for assuming that their visibility 
follows the same simple laws which hold for point 
sources of light. The experiments show in effect 
that the visibility of objects subtending angles up 
to about ten minutes of are follows the same law 
as for point sources, viz., the brightness must be 
inversely proportional to the square of the dis- 
tance, but that for objects above this the bright- 
ness is proportional simply to the distance. It is 
the larger size of objects which have generally to 
be discerned when driving at night, and it there- 
fore follows that for equal visibility at twice the 
distance the equivalent candle-power of a head- 
light must be increased eight times, and not 
sixteen times, as would be the case if the inverse 
square law held. Thus it follows that the useful 
range of a motor headlight is proportional to the 
cube root of the intensity of the beam. 
The visibility of objects viewed in the presence 
of glaring lights is a matter which has been much 
discussed, but the reason for the discomfort and 
NO. 2380, VOL. 95 | 

annoyance which is felt when one has a light in 
one’s eyes is rather obscure—except in such gross 
cases as the dazzle produced on the dark adapted 
eye by motor headlights. Experiments show that 
at ordinary illuminations it is very difficult to 
detect by measurement any diminution of ability 
to see detail in the presence of bright lateral 
lights, although there is little doubt about the 
discomfort caused by them. When, however, the 
surface brightness of the objects to be dis- 
tinguished is very low there is, when lateral lights 
are present near to the object, a marked falling 
off in the ability of the eye to distinguish the 
slight contrasts which exist over such surfaces, and 
detail appears in consequence obliterated. A room 
viewed under these conditions might have to the 
eye somewhat the appearance of an under-exposed 
negative in which only the high lights and strong 
contrasts show up. It cannot be said, however, 
with certainty that herein lies the evil of glaring 
lights, and the discomfort experienced with them 
may very well be psychological in character. 
The discrimination of detail in certain circum- 
stances depends almost entirely either on shadow 
or on the direction of the incident light. Where 
the surface of an object to be viewed is uneven, 
but uniform in colour, the only way in which the 
unevenness can be shown up is by differences in 
the illumination of such surfaces caused by the 
different angles which they present to the incident 
light. If the unevennesses are very deep they will 
be shown up by the shadows which are thrown 
by the raised portions on to the surrounding sur- 
faces. For instance, the embossed lettering used 
on some notepaper depends entirely on this action, 
and a sculpture in bas-relief must obviously 
present a very different appearance according as 
it is illuminated by unidirectional light at glancing 
incidence or by light from a large source striking 
it mainly at normal incidence. 
The question has a wide practical application in 
the manipulation of self-toned fabrics, viz., uni- 
coloured fabrics with no natural contrasts. A 
person doing needlework with such materials 
depends for the discrimination of the detailed 
strands of the fabric on the small shadows cast 
by one strand on to the next and on the varying 
brightness over the curved surfaces of each indi- 
vidual strand. Both these factors depend on the 
unidirectional character of the light incident on the 
material, and the inefficiency of thoroughly 
diffused, viz., indirect light, for such work is 
most marked. The writer has, for ten years, used 
indirect lighting for domestic use, and for a long 
time ridiculed the assertion which was often made 
that it was a most unsatisfactory light by which 
to do needlework, and particularly darning. 
After experiment, however, it is clear that the 
contention is sound, and that there is nothing so 
good as a_ unidirectional light giving harsh 
shadows for the discrimination of detail in needle- 
work of all kinds. Indirect lighting, with its soft 
shadows, is an ideal light for domestic use, except 
in respect of this one particular. 
C. C. PATERSON. 
