ADVERTISING ILLUSTRATION
An example of commercial photography
THUMB-PRINT ON DARK WOOD
Finger-prints are made visible by dusting with a fine powder, and are photographed with a special detective camera
Though the photographic plate thus extends the usefulness of the microscope, this is not the limit of its value in this respect. Light is transmitted by waves, similar in some ways to waves in water, the light waves being disturbances of the light-bearing ether, an invisible, imponderable substance of zero density and infinite elasticity, which pervades all matter. (It must be understood that the ether has never been observed nor its actual existence proven; it is, however, a necessary assumption for the satisfactory explanation of the observed phenomena of light, so far as our present knowledge extends.) The distance from the crest of one wave to the crest of the next is known as the wave length, the lengths of the various light waves having actually been measured. The human eye is sensitive only to waves between about four-ten-thousandths and seven-ten-thousandths of a millimeter in length, a millimeter being about one-twenty-fifth of an inch, and an object is invisible in the microscope if its diameter is less than half the wave length of the light by which it is illuminated, since in that case the light waves bend around the object and meet on the other side. We cannot, therefore, see objects whose diameter is less than about two-ten-thousandths of a millimeter. But the photographic plate is sensitive to shorter waves than the eye; these waves are known as the ultra-violet. By illuminating the microscope stage with ultra-violet light it therefore becomes possible to photograph objects so small that they must forever remain invisible to the naked eye, unless, indeed, the progress of human evolution brings with it increased sensitiveness to the shorter wave lengths. In this connection it is interesting to note that there are organisms so small that they cannot be made apparent to us even by photography, though we are made aware of their existence by inductive reasoning from their observed effects.
NEWSPAPER ILLUSTRATION
Enlarged to show half-tone screen
In the case of some objects, a fuller knowledge of their character is gained if they are examined in a manner somewhat different from that usually adopted. One of the photographs given herewith shows the effect obtained by what is known as “dark ground illumination.” Ordinarily, the light by which a microscopic object is examined passes through the slide, so that an opaque object is really seen only as a silhouette, but in dark ground illumination an opaque background is placed behind the object, and the light is allowed to fall on it from the sides. The object is thus made visible by the light that is refracted (that is, bent) into the lens of the microscope. In the present instance the effect seen by looking into the eye-piece was wonderfully beautiful, the crystals glowing with a brilliant yellow light against an intensely black ground.