The daily Alaskan., 1910-04-05
LIGHT AND THE EYES
We Do Not Always See Things
 as They Really Are.
CoLoRS WE CANNOT DISCERN

let-Soms Results That Have Been
 Attained With the Camera.
 Those of us who are fortunate enough to possess normal eyes are apt to feel fnstincuvely that we see things as they are. This Is a mistake. The appearance of the world at large is merely the result of the circumstance that the human eye perceives only a comparatively small part of the total radiation which comes from the sun or is given out by lamp
 To make this little clearer we may consider one of the oldest experiments in optics, the passage of ray or hgnt through a prism.
 When a beam of sunlight passes through prism of glass such as is often found on old fashioned chande- Hers the rays are spread out, forming a spectrum. which we see as a rain how colored band upon the wall The colors of the spectrum merge into one another by msensible gradauons' though the older physicists were in the habit of recognizing seven-reu, orange. yellow, green, blue, indigo and violet
 These coIors which the eye perceives are by no means the only ones present in the spectrum. If we perform the experiment in a dark room and place a photographic plate in such a position as to receive the image O. the spec trum we find that it extends to con- siderable distance beyond the violet.
 In other words. there is a region De- voDd the violet which the eye cannot see but which the sensitive plate is able to photograph. This region is termed the ultra violet, and the light from virtually all sources contains a greater or less amount of this ultra violet light
 The only difference between this ul- tra violet light and ordinary visible Trht is in the length of the wave,
 which is. in fact, the only difference between the different colors of the visible spectrum. If, instead of using
 a glass prism for forming our spec
 rum. we use one cut from quartz crystal we find that the ultra violet
 snactrum is very much longer, which
 shows us that the shortest waves or light are not transmitted even by the
 the other the appearance of things
 would appear black, our windows
 would be as opaque as sheets of iron, and polished silver would resemble
 anthracite coal In other words, thiEgs 1 appear as they do merely from the cir ! cumstance that in the process of evO- | lution our eyes have developed a senSi-
 bility to a certain region of the total | spectrum of the light which comes to us from the sun.
 If we have used an ordinary photo graphic plate in our experiment we shall find that 1t has received no i'm ] pression from the red and yellow parts of the spectrum and very little from l the green. The plate therefore suffers from the same defect as our eye It is sensitive only to a limited region. Recently however. means have been found of rendering plates sensitive to tho entire visible spectrum and to a certain extent to the mira red or the extension beyond the red analogous to the one beyond the violet
 With a photographic plate rendered sensitive to the entire spectrum we could obtain an idea of how things would appear if our eyes were sensi- tive to regions other than the one adopted by nature by excluding from the lens of the camera all rays other than the ones with which we wish to operate. This is done to a limited ex- tent every day by all photographers who employ color screens in their work. and 1t is well known that in- creased contrast can often be obtained in this way.
 If, for example, we attempt to pho tograph white clouds against a blue sky without the use of such a screen We get scarcely any trace of the clouds at all. whereas if we place a yellow glass in front of the lens the clouds come out snow white on a very dark background.
 We can see this same effect by view ing the clouds first through a piece of deep blue glass, which is what the plate sees" and then through the yellow plate.
 At my laboratory at East Hampton, N. T., l have pushed this principle to the extreme and have obtained photo graphs both by the mira red and the ultra violet invisible Robert Will ! liam Wood in Century.