388 Transactions of the Canadian Institute. [Vol. V^II. 



is to obtain the correct ratios of exposure through the three filters. 

 Remembering that in the synthesis equal quantities of the reproduction 

 or printing colours gave white or grey, it is evident that objects free 

 from colour must be represented by equal density in the three 

 negatives. The ratios of the exposures made, in a steady light, on such 

 a test object as a piece of crumpled white blotting paper, must be so 

 adjusted that the density of the resulting negatives, after development 

 for the same time in one tray, will be equal. Each batch of emulsion 

 requires a test of this nature as the relative colour sensitiveness of 

 different batches varies slightly ; for example, in one emulsion the 

 ratios of exposure were found to be red, 60 ; green, 8 ; blue, i ; and in 

 another red, 30 ; green, 6 ; blue, i. 



The exposure through the red filter is about five hundred times the 

 normal exposure without a filter, so that, using ordinary apparatus, and 

 allowing for the changing of plate holders, filters, etc., a set of three 

 exposures, under the best possible conditions, of a well lighted outdoor 

 subject will require at least a minute ; while for indoor exposures ten 

 minutes and upwards, depending upon the light, will be required. Any 

 movement of camera or object, any error or omission in the sequence of 

 operations, about fifteen in number, or any change in the quality of the 

 light, unless correctly allowed for, renders all three negatives useless. 

 The use of a special camera, in which all three exposures are made 

 simultaneously, considerabl}' reduces the time of exposure, and, by 

 lessening the number of operations, diminishes the liability to error; 

 but such a camera is expensive to purchase and troublesome to keep in 

 order. 



It is advisable before development to mark or letter a corner of the 

 film on each plate, as it will be frequently found difficult to distinguish 

 the negatives from one another. The plates are developed together in 

 one tray in a metol developer, without bromide ; with any other 

 developer, especially hydroquinone, the images will not appear equally, 

 and a different range of gradations will be obtained giving faulty 

 results. Short development producing soft delicate negatives is 

 required ; over development bleaches out the light tints of colour. 

 Under exposure causes excessive colour contrasts, while over exposure 

 weakens the contrasts. The negatives are fixed and washed in the 

 same manner as ordinary negatives, and after drying are ready for 

 making the transparencies. 



For positive synthesis, whether for triple projection or the kromskop, 



