164 



SCIEXCE-GOSSIP. 



dividing the beam of light into three portions, is 

 necessary. The Repeating Back is. however, the 

 general favourite of the amateur, as it is so simple 

 in operation and occupies so little space. 



In taking the colour negatives, one point only 

 has to be remembered- The negatives through the 

 three filters will require different exposures, for. as 

 we saw (page 111), a white object must be repre- 

 sented by equal density in all three negatives. 

 With each Repeating Back the makers furnish the 

 relative times of exposure for each filter : for in- 

 stance, the red filter negative may take nine 

 seconds, the green three seconds, and the blue 

 three seconds, and whatever exposures are necessary 

 for our subject they must be given in this pro- 

 portion. Different batches of emulsion may vary 

 slightly in colour-sensitiveness, and it is very easy 

 for the amateur to test his ratio by photographing 

 such an object as a sheet of white blotting-paper 

 roughly crumpled and pinned upon a black surface. 

 A single negative, exposed and developed, will 

 show at once if any alteration is required- 

 It only remains then to describe the method of 

 printing the separate sensations, and mounting 

 them in superposition. The minus red or greenish- 

 blue print is made upon a gelatino-bromide of silver 

 emulsion plate, and after development the silver 

 deposit is replaced by ferro-cyanide of iron, the 

 metallic silver acting as a mordant. This sub- 

 stitution product is the particular tint of greenish- 

 blue required, and the image is a very delicate one 

 of great beauty. The prints from the green and 

 blue filter negatives are printed together upon -a 

 strip of special fil m of thin transparent celluloid 

 which has undergone a process of air-drvine. in 

 order to ensure freedom from unequal contraction. 

 The celluloid is coated with a soluble gelatine film 

 containing a trace of bromide of silver. The film will 

 keep indefinitely, and it is rendered sensitive for use 

 by im m ersion in a solution of a chromate salt in the 

 manner of carbon tissue. The gelatine film being, 

 however, very much thinner than ordinary carbon 

 tissue, all the usual difficulties of drying, insensitive- 

 ness, etc., are absent, and as the celluloid side of the 

 film is placed in contact with the film side of the 

 negative, so that the exposure takes place through 

 the celluloid film, no tr an sfer with its attendant 

 difficulties is necessary. The image is a visible 

 one and can be exa min ed from time to time, the 

 exposure being complete when all the details are 

 distinctly seen, as a light brownish-grey image, 

 very similar to an undeveloped platinotype print. 

 The printed film is then washed out in warm water, 

 the trace of silver bromide enabling us to see 

 when the development is complete, the image held 

 over a dark surface appearing as a delicate white 

 print in low relief. The silver bromide, having 

 served its purpose, is dissolved out by a solution of 

 hypo-sulphate of soda, and the resulting low relief 

 in clear gelatine washed and dried. The two 

 prints which have so far been treated together are 



now cut apart, and the print from the green -filter 

 negative stained in a pink dye-bath, so as t _ 

 pink print which allows red and blue light to pass. 

 The print from the blue-filter negative is stained in a 

 yellow dye-bath so as to get a yellow print, which 

 will allow red and green light to pass. When the 

 two prints are dry. they are placed in superposi- 

 tion upon the greenish-blue print and our 

 is finished. As the two film prints are r- 

 prints, in consequence of our printing thro v. _ 

 film, and the greenish-blue print is a direct one. 

 the two most important components of the triplet 

 — the greenish-blue and pink prints — are mounted 

 in actual contact, and as the third print, the yellow, 

 is only separated from them by one thickness of verv 

 thin celluloid- the finished print beha 

 way as a single picture. 



Lantern slides made in this way are as trans- 

 parent as the best h a n d-coloured slides, although. 

 of course, in finitely superior in delicacy and accu- 

 racy of colouring. They may be shown in an 

 ordinary lantern, without the necessitv for anr 

 extra attachment : the mixed jet limelisht givine 

 perfectly satisfactory discs up to about VI : 

 diameter. With the electric-arc light pictures of 

 20 feet diameter or more can be easily shown. 

 For projecting pictures naturally, the white- 

 available should be used — preferably the electric 

 arc, but the mixed oxyhydrogen limelight, if a small 

 jet is used, will give a very satisfactory light, 

 although the pictures will be a little warmer in 

 colouring than they would be as seen by da . 

 For home exhibition, on a disc up to two feet ::: 

 diameter, a double acetylene-gas burner will prove 

 quite satisfactory. , 



Before concluding it is desirable to correct an 

 oversight that appeared, on line 6 from the top of 

 page 112. The sentence shoald read: "Each of 

 these colours therefore transmits two, and absorbs 

 one, of the three primary colours used by Clerk- 

 Maxwell to form white light." 



London, October 1900. 



The Xobel Bequest. — Just before going to 

 press we have been furnished through the Board 

 of Education, which has received them from the 

 Foreign Office, with copies of the official statutes 

 and regulations of the Nobel Bequest. The large 

 amount of annual interest accruing will be divided 

 equally into five annual prizes. Three of these 

 apply to scientific research. They are : (1) To the 

 most important discovery or invention in the do- 

 main of the physical sciences; (2) to the most 

 important discovery" or improvement in chemistry ; 

 (3) to the most important discovery in physiology 

 or medicine. The competition is open to the 

 world, and, it is believed, will be held for the first 

 time in 1901. Next month we shall probably give 

 further details with regard to these valuable 

 ; :::.? 



Two New Menob Plaxets were photographic- 

 ally discovered by Professor Max Wolf and Herr 

 Schwassmann, of Heidelberg, on 15th and 21st of 

 September. 



