1900-1901. | Orthochromatic Photography. IQI 
action. The light we receive from the sun is called white. 
Its passage through clear space is invisible, the presence of 
the vibrations producing it only becoming sensible to the eye 
when they either penetrate it directly or are reflected from 
some surface on which they fall. Tyndall, in order to 
demonstrate the invisibility of light rays traversing trans- 
parent media such as air, allowed a pencil of light to pass 
through a box, the interior of which was blackened. The 
path of the light was easily seen, being clearly marked out 
by the small particles of dust which were illuminated in its 
track. He then took a lighted spirit-lamp, which burns 
with a smokeless flame, and applied the flame to the stream 
of light passing through the dark box. The particles of 
dust in and around the flame were at once consumed, and 
the light, having nothing to reflect it, disappeared from the 
heated portion, the effect produced being the appearance 
as of thick black smoke arising from the smokeless flame. 
When a ray of light passes from a rarer to a denser 
medium, it becomes refracted or bent from its original course. 
In passing through a denser medium such as glass, if the 
sides of the glass are parallel, the ray on emerging from the 
other side is bent back again to a course parallel to its 
original direction and otherwise suffers no change. If the 
sides of the glass are not parallel, as in the prism, then we 
see that the ray of light, on leaving the second surface, is 
bent still more from its original course than at the first sur- 
face, and pursues an entirely different direction. If this 
refracted ray be now intercepted by a white screen, we shall 
see, not a spot of white light, but a somewhat lengthened 
coloured band—an old but beautiful experiment, showing 
that white light is made up of rays which give to the eye 
different colours according to their refrangibility (fig. 1); 
the least refrangible giving us the sensation of red, and, 
as the refrangibility increases, passing through orange to 
yellow, yellow-green to green and blue, indigo, and finally 
violet. Experiment has shown that the red rays have a 
longer wave-length, and the violet a shorter, the wave-length 
gradually decreasing from the red to the violet. Besides 
those rays that are rendered visible to the eye there are 
others beyond the visible limits of the spectrum. The infra- 
