m 



November ^o, 1899J 



NATURE 



«o5 



does it differ? It seemed possible that the peculiarity lay in 

 the nature of its radiation. To test this a prism was placed 

 before the lens of the camera, which broke up the image of 

 the spark into a series of spark images of different colour. 

 The plate was exposed to the flashed spectrum of a single 

 spark, then removed from thei camera and exposed to the 

 candle light, and developed. If the reversing effect was due to 

 any peculiar radiation or wave-length we should find the reversal 

 at that part of the spectrum where the effective radiation 

 belonged, say in the infia red if the reversing power lay in long 

 waves given out by the spark. It was found that the entire 

 spectrum came out lighter on the negative than the fogged 

 background. A second plate was exposed to the spectrum flash, 

 then slightly fogged, and a second spectrum impressed on it in 

 a different place. On developing, one spectrum came out light 

 and the other dark. Clearly the effect does not depend on wave- 

 length. It then occurred to me that the time-element might 

 enter into the problem. The light of the spark is over in about 

 1/500CX5 of a second, and it did not seem impossible that a 

 bright light of exceedingly short duration might act quite 

 differently on a plate from a weaker light of longer duration. 

 This may be tested in a variety of ways. We may open the 

 lens wide, impress the image of a single spark on the plate, and 

 then stop the lens down and superimpose a number of spark 

 images sufficient to make the total exposure the same in each 

 case. This was the first method which I tried. In order to 

 compel the successive sparks to pass over the same path, that 

 their images might be superposed, I shut them up in a capillary 

 tube. With the lens open wide enough to give the maximum 

 reversing action, I passed a single discharge through the capil- 

 lary. Stopping the lens down to one quarter of its former 

 aperture, four discharges were passed through the tube. The 

 plate was then fogged in the usual manner, and on development 

 the single discharge was reversed, but the composite one was 

 not. 



Fig. 2 is from a plate showing this effect. The upper images 

 are those of single discharges through the capillary, with dif- 

 ferent apertures of the lens ; the lower images are those of 

 double or triple discharges through the same tube. The left- 

 hand side of the plate was exposed to the candle light for dif- 

 ferent amounts of lime, by moving the screen over small dis- 

 tances during the exposure. Only the single discharges reverse, 

 though the density of the images on the unfogged portion of the 

 plate is the same. 



This was very strong evidence that the duration of the illu- 

 mination was the important factor. Some years ago I measured 

 the duration of the flash of exploding oxy-hydrogen, finding it 

 to be about i/ 12000 of a second. Possibly the flash of such an 

 explosion would duplicate the effect. I exploded several glass 

 bulbs filled with electrolytic gas, but found that the action was 

 the same as that of ordinary light, it being impossible to get 

 any reversal. The flash evidently lasted too long, or there still 

 remained some undiscovered factor. 



The difference between the action of spark light and the light 

 of the oxy-hydrogen flash is shown in Fig. 3.^ Plate "a" shows 

 the effect of the explosion flash. Squares i and 2 received the 

 light from an exploding bulb, the rest of the plate being covered. 

 Squares i and 3 were then exposed to the light of the candle. 

 Square i, which has received the light from both sources, is the 

 brightest, that is, the effects are additive, there being no reversal. 

 Plate "b" shows the action of the light from the spark. 

 Squares i and 2 were illuminated by the spark light, then 

 squares 2 and 4 were exposed to the candle. In this case, square 

 4, which was illuminated by the candle, is brighter than square 

 2, which received both the spark light and candle light. In this 

 case the effects are not additive, there being reversal. 



To demonstrate conclusively that the time-factor was the only 

 one, it was necessary to secure an illumination independent of 

 the electric spark, and of as short duration. This was accom- 

 plished in the following manner : A disc 30 cms, in diameter 

 was furnished with a radial slit i millimetre wide near its 

 periphery, and mounted on the shaft of a high-speed electric 

 motor. A second slit of equal width was arranged close to the 

 rim of the disc, in such a position that the two slits would be in 

 coincidence once in every revolution. This second slit was cut 

 in the wall of a vertical chute, down which a photc^raphic 

 plate could be dropped. By means of a large convex lens of 

 short focus, an image of the crater of an arc-lamp was thrown on 



1 The details in this figure, and in two others sent by Prof. Wood, are 

 too indistinct to be reproduced satisfactorily. — Ed. Natuke. 



the point of coincidence of the slits. The intensity of the 

 illumination transmitted by tlje slits when in coincidence was 

 almost sufficient to char paper. The motor was now set ii> 

 motion, and a plate dropped down the chute. On developing, 

 this plate, three images of the slit appeared, not at all over- 

 exposed, though the plate was the fastest obtainable, and the 

 intensity of the light while it lasted comparable to that at the 

 focus of a burning glass. By measuring the distance between 

 the images and the vertical distance through which the plate had 

 fallen, it was an easy matter to calculate the speed of rotation, 

 which was found to be sixty revolutions per second, the air frictioik 

 of the disc preventing higher speed. The duration of the exposure 

 will be the time occupied by the rim in travelling a distance 

 equal to the width of the slit, or i mm. This was found to be 

 1/55000 of a second, about that of the spark. The crucial 

 experiment now remained. A second plate was dropped, and,, 

 before development, was exposed to the light of the candle. 

 The images of the slit were most beautifully reversed, except at 

 the centre, where the light was too intense. It seems, then, 

 that we are justified in assuming that the action of an intense light 

 on a plate for a very brief time-interval decreases the sensitive- 

 ness of the plate to light. It is curious to contrast with this 

 effect the fact that exposure to a dim light for a moment or two 

 appears to increase the sensibility by doing the small amount of 

 preliminary work on the molecules, which seems to be necessary 

 before any change can be effected that will respond to the 

 developer. 



I am not prepared to say what the nature of the change 

 effected by the flash is. Possibly some one familiar with the 

 theory of sensitive emulsions can answer the question. I have 

 tried using polarised light for the reversing flash, and therk 

 fogging one half of the plate with light polarised in the same 

 plane, and the other half with light polarised at right angles- 

 to it. As was to be expected, there was no difference in the 

 effects. R. W, Wood. 



Physical Laboratory of the University of Wisconsin, 

 Madison, Wisconsin, October 20. 



Experiments on the Floral Colours. 



In 1837 the illustrious Berzelius wrote: "The red pigment 

 of several kinds of berries has generally been regarded as a 

 blue pigment reddened by an acid. This is not the case with 

 all berries. I have examined the pigment of Prunus cerasus 

 and of Eibes nigrum, which contain the same pigment, and this 

 is not blue. Probably this has been surmised from the circum- 

 stance that the sap of these berries gives a blue precipitate with 

 acetate of lead, but these precipitates are malate and citrate of 

 lead, wherewith the pigment is combined." He found that, 

 after separating these acids from the colouring matter, the latter 

 yields a green and not a blue precipitate with acetate of lead ; 

 and, moreover, when to its aqueous solution a little milk of 

 lime is added sufficient to saturate all the free acid, the super- 

 natant liquid is red and not blue, which latter it would be if 

 its natural colour was blue. He arrives at similar conclusions 

 with regard to the red pigment of the autumn leaves of cherry,, 

 red currant, &c. 



On the other hand, Julius Wiesner, of Vienna, in 1862 and 

 1872, by a series of experiments, endeavoured to prove that 

 the compounds of anthocyan — i.e. the blue and red pigment of 

 flowers, with lead, alkalis, &c, — are always blue, and it is only 

 when anthocyan is present in the cell-sap simultaneously with 

 a substance which is coloured yellow by alkalis, &c., that it 

 passes by the latter body into green, which thus arises as a 

 mixed colour. He found that by completely washing out (as 

 he thought) this latter body from the petals by warm dilute 

 hydrochloric acid, and then immersing them in solutions of lead 

 and iron salts, they became intensely blue ; hence he was led 

 to conclude, contrary to Berzelius, that the original and actual 

 colour of anthocyan was blue and not red. 



During last summer I have performed a series of experiments 

 on a number of flowers, with a view of settling the question iiv 

 dispute, as above set forth. In the first place, it was deemed 

 advisable to observe the effect produced in each case by im- 

 mersing the fresh petal into ether saturated with ammonia. The 

 results were as follows: — (i) Petals which became blue, «.^. 

 pseony, pink campion, deep red garden rose, sweet pea, vetch, 

 mallow, balsam, geranium, fuchsia, scarlet rhododendron, 

 crimson flax, blue centaurea ; red daisy, periwinkle, lady's smock, 

 became bluish-green. (2) Petals which became green, e.g. 



NO. 1570, VOL. 61] 



