Description. The Kelsh plotter is a projection-type instrument employing contact diapositives.

A Kelsh model differs in appearance from a multiplex model at the same scale only in that the definition, and the illumination, of the former tend to be better. Detail and topography are taken off by means of a multiplex tracing table. Setting up and operation do not require further explanation.

It is to be noted that, since it is a two-projector instrument, it is not necessary to provide six degrees of freedom to each projector, and accordingly controls are simplified. The model having been set up and scaled, is horizontalized by the bar which may be moved on a three-point system either independently of, or with, the frame footscrews.

Model scale is variable between 4½ and 5½ times contact scale. Data of p. 127 could be taken as referring to a Kelsh working 4 times enlargement from 6-in. 9 by 9 contact diapositives.

In the 6-in. Kelsh as manufactured by The Instruments Corporation, PD is variable from 27 in. to 33 in.—700 mm. to 850 mm. Corresponding instrumental air base variation is 16 in. to 20 in.—400 mm. to 500 mm.—for 60% forward lap.

The projectors may readily be adapted for 8¼ in. and other lenses.

Kelsh System of Illumination. The problem of providing illumination for a full-size diapositive has been mentioned briefly, p. 127. Von Gruber(i), p. 304, points out that, compared to an image viewed by transmitted light, in projection upon a screen only the one twenty-thousandth part of the available light may reach the eye.

It is understandable, therefore, why previous designers of instruments employing contact scale transparencies rejected simple solution by means of direct optical projection. In spite of its other manifest advantages, the illumination problem appeared to be insurmountable. In the fine instruments of Zeiss, Wild, Pouvillier, the image is viewed by transmitted light, and spatial intersection recovered by optical and mechanical linkage of great ingenuity, and manufactured necessarily to almost superhuman limits of precision, that the space rods may accurately parallel the paths of the two corresponding rays of light.

In a full-size projector, it is evident that illumination is required, not of the whole format, but only of that part which is being examined—the area of the tracing table platen.