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Distilled

lens, distortion and fig

DISTILLED WATER.—(Formula molecular weight, z8.)—Pure water obtained by distillation. In all delicate and important photographic operations distilled water should be used. Pure distilled water should give no precipitate with AgNO, showing absence of any chloride; nor with ammonia oxalate, which would otherwise show the presence of lime; nor with barium chloride proving the absence of sulphuric acid. If a drop of permanganate of potassium or Condy's fluid be dropped in the water it should give a pink tint, thus proving the absence of organic matter.

DISTORTION.—When the image formed by a lens upon the focusing glass of the camera does not coincide with the laws of perspective it is said to be distorted. There are two kinds of distortion—one in which objects are exaggerated in dimensions quite contrary to the laws of perspective. A wide-angle lens is guilty of this distortion, and therefore should never be used unless absolutely necessary to include a view which cannot otherwise be made. The other kind of distortion is produced in most lenses by the refraction of the oblique eccentrical pencils of light by the margin of the lens. Distortion by divergent lenses is just the reverse of that by

convergent lenses. Thus the lines Fig. x64 would be distorted as in Fig. 165 with a divergent lens, but with a convergent one they would appear as in Fig. z66.

The position of the diaphragm in a single lens alters the distortion also. If placed in front of the lens the distortion is as in Fig. 165, but if behind the lens it would appear as in Fig. i66.

It is therefore clearly seen that if we use two sym metrical lenses, and place the diaphragm between, the dis tortion of one is counteracted by the distortion of the other, and the lens is said to be rectilinear. Upon this principle all the non-distorting doublets are manufactured.

The distortion of a single lens is not great unless its focus be very long, and it is only noticeable in taking buildings near to, which should never be done except with a correct lens. Dallmeyer has recently introduced a rectilinear single lens which may be considered a great achievement. See also Lens.