This section is from the book "Complete Self-Instructing Library Of Practical Photography", by J. B. Schriever. Also available from Amazon: Complete Self-Instructing Library Of Practical Photography.
Introduction. Landscape lantern-slides are rarely seen without clouds, as they add materially to the general appearance of the view. There is nothing that will mar the appearance of a scene so much as a flat, chalky white sky, or even a mottled effect in the sky portion.
933. Wherever possible, one should aim to secure clouds in the original negative with the landscape, for by so doing a large amount of subsequent work will be obviated. By employing the orthochromatic plate and a ray-filter the securing of clouds in landscape negatives is an easy matter, but when occasion arises where clouds do not exist in the negative or print from which the lantern-slide is being made, some means must be employed to combine clouds with the foreground, thus producing a complete and pleasing slide.
Sky Denser Than Foreground. There are cases where cloud portions of landscape negatives are so much more opaque than the foreground, that they are not well defined in the slide. In such a case the sky portion of the landscape negative should be carefully reduced with Potassium Ferricyanide or Persulphate of Ammonia (see Reducing, Volume II) until the clouds assume an opacity of equal printing value with the landscape portion. There are cases in which the sky of the negative is only a trifle denser than the landscape, in which case it is not desirable to reduce that portion. It would be advisable, under such conditions, when making the slide, to shade the landscape so that the denser part (the sky) of the negative may have a few seconds additional exposure.
Introducing Clouds Into Slides By Reduction. There will always be a certain number of slides which will require the introduction of clouds from other negatives. There are two methods generally employed, whereby the best of results may be secured: The clouds may be printed on a separate lantern plate from a specially made cloud negative, and this cloud slide used as a cover-glass, or, the clouds may be printed on the same plate as the landscape portion by double printing.
Cover-Glass Method By Reduction. This method requires a variety of cloud negatives from which to select, in order to obtain a suitable cloud effect for any particular landscape. Chapter XIII (Photographic Printing), of Volume III, gives detailed instruction as to the various methods of producing cloud negatives, and also treats upon the various forms of clouds. Persons desiring to secure a representative collection of cloud negatives should study the instruction given in Volume III. It will be advisable to make prints from each of the cloud negatives, and mount these in an album, placing underneath each print the time of day the negative was taken, and the direction of light. If this is done there will be no danger of contradictory cloud effects being used in making the slide. If any of the landscape shows the cloud negative it may be blocked out by sticking some non-actinic paper upon the back of the negative.