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July 3, 1885.J THE PHOTOGRAPHIC NEWS. 423 When a negative to be reduced is thick on one end and thin on the other, the thin portion can often be shaded with success. I have even thrown a handkerchief over the thin end, and thereby obtained a more even illumination. Until some better light is provided, it will be necessary to make a double set of slides for home exhibitions, one suited for oil, and the other for the lime light. I have some subjects that I cannot develop thin enough for oil, and retain any pluck ; yet if the development is continued, the transparency becomes practically worthless. I employ two cameras facing each other ; in one the negative to be reduced is placed in a kit in the space occupied by the plate-holder, and the lens front is removed. The lens of the copying is inserted in this operture, and the space between the two cameras is covered by a black cloth; this plan I find works very successfully, and avoids the necesity of using a copying camera. In focussing I employ a magnifying glass. The making of lantern slides is to me extremely fascinating, and if I have imparted any information which can be utilised by others, 1 shall feel that my experience has not been in vain. ISOCHROMATIC PHOTOGRAPHY. BY FRED. B. IVES.* For the purpose of better illustrating the capabilities of my process of isochromatic photography 7 , I have recently made a large number of photographs of a colour-scale, which is made up of pieces of woollen cloth dyed red, scarlet, yellow, green, blue, violet, magenta, &c. The exposures were made through colour- screens, which were carefully selected by the aid of the spectro- Red. Scarlet. Yellow. scope, in order that it might be known in each case what kind of light was transmitted. I have not had time to make a set of lantern positives, but will show some of the negatives on the screen, and also one positive, made from prints which have been arranged for com parison. The first negative is an ordinary photograph of the colour scale, in which blue, violet, and magenta are the only colours which photograph with considerable intensity. The second photograph was made with an eosine-stained plate, exposed through a yellow screen. Light blue, green, yellow, and light yellow-brown are the only colours which photograph well. The third photograph is by my isochromatic process, and all of the colours come up in approximately the true proportions of their brightness. The fourth is one of my chlorophyll plates exposed through a scarlet screen, and brings out with remarkable intensity all those colours which have usually been regarded as most non-actinic ; but green and blue come out like black. i In the lantern positive, seven squares are shown from each I negative, arranged for comparison. The first row is from the ordinary photograph, the second is from the eosine plate exposed through a yellow screen, the third is from the isochromatic photograph, and the fourth from the chlorophyll plate exposed through the scarlet screen. These photographs tell their own story so far as regards the capacity of the processes; but my investigations have revealed some remarkable facts, which I will now state briefly, without entering further into the details of my experiments:—■ 1. Although collodio-bromide emulsion plates, stained with blue-myrtle chlorophyll alone, are capable of photographing all Green. Blue. Violet. Magenta. coloured objects in the true proportions of their brightness, they are far more sensitive to the extreme dark red of the spectrum, i below the absorption band of chlorophyll, than to either the orange, yellow, or green. 2. All red, orange, and yellow objects reflect the dark red light to which chlorophyll plates are so sensitive, and bright yellow objects reflect as much of this light as red ones. Yellow 1 objects photograph lighter than red ones by this process, because I they reflect two kinds of light to which the plates are sensitive, while red objects reflect only the one kind. 3. The sensitiveness of the chlorophyll plates to spectrum ♦ From advance proof sheets of the Journal of the Franklin Institute. Read at the meeting of the Institute, 'Wednesday, May IB, 1885. yellow and green may be greatly increased by treating them with the tea organifier, which also nearly doubles the general sensi tiveness, but without appearing to alter the efiect in photo graphs made through a yellow screen. If a green screen is used, of a shade which does not transmit the dark red of the spec trum, the resulting photograph is better when the tea organifier has been used, and does not then differ in any respect from one made by exposing an eosine-stained plate through the same screen. 4. The dark red of the spectrum passes freely through a solution of chlorophyll, and through ruby and orange glass, but is absorbed by common green glass, or by solutions of sulphate I of copper or prussian blue.
