604 



KNOWLEDGE • 



[May I'J, I8&1; 



Prepare anotlicr card i)y occupying two holes with discs 

 •of bluo and yellow, each having one, and the third hole 

 with both colours superposed. The result of this super- 

 •j>osition is a green, as it would he with pigments, and it is 

 interesting to compare this green with the approximate 

 white ol)tJiined by superposing the tints by means of the 

 <louble inuige prism. A similar card prepared with crini.son 

 and green separate, and also the two combined, are like- 

 wise instructive. The actual superposed combination of the 

 gelatines effects an orangey nioditieation of the crimson, 

 but when the superposition is made by increase of the 

 jirism, yellow is the result. Other combinations by super- 

 position and by prism should also be compared. 



After the above exporinionts, gum three strips of white 

 jiaper, about one inch long by half-an-inch wide, on a 

 .s<iuare of lilack cardboard, a little way apart. On one 

 slip fasten (which can be done by slightly moistening) a 

 •square of blue gelatine and one of yellow beside it E.x- 

 pose to full white light, and cause superposition by the 

 prism. The tint obtained is a chocolate grey. Red and 

 crimson thus tinted give orange ; crimson and green, dark 



K^py. 



The effect of a double image prism, by dividing one 

 c.iloured ray into two, is to lessen its force, and the effect 

 of adding, by reflection or transmission, a strong white 

 light to any colour is to thin it out, or completely subdue 

 it. Stick a .small square of crimson gelatine in the middle 

 of a sky-blue disc li.xed in a black cardboard square ; hold 

 it against a white cloud ; duplicate the image of the crim- 

 son square with the prism ; such image is bluish-grey. 

 Jlake portions of the two images touch ; the crimson, modi- 

 tied by the blue, at once appears. 



In studying these various effects, the exact conditions 

 of each experiment m\ist be noticed. Thus, dealing with 

 spectrum colours is employing rays of light of certain re- 

 t"rangil)ilities, according to the dispersion obtained. Pig- 

 fliients are either opaque or transparent. The action of an 

 opaque pigment is to absorb certain rays composing white 

 light, and to reflect the residue. If such a pigment is seen 

 iu strong white light, the quantity thereof which it reflects 

 acts in proportionate diminution of its normal colour. In 

 viewing gelatine pastes held up against a white cloud, the 

 colour seen is the residue left after its absorption of the rays 

 that cannot pass through it. If one film is imposed over 

 -another, and both are held up to the light, the first film only 

 transmits to the second the ravs that remain after its 

 -absorpti\e powers have been exercised. If a transparent 

 tint is superposed upon white paper, that paper reflects 

 through it the rays which the coloured film has not absorbed, 

 and subjects them to a fresh action of the film as they pass 

 for the second time through it. 



To show what white, or approximately white light can 

 do with a strong colour, throw a full red on a white screen 

 "ith a magic-lantern. Then with another magic-lantern, 

 or a common bull's-eye lantern, placed nearer, and thus 

 i-usting an intenser light, overpower the coloured light. A 

 liand with outspread fingers, held so as to obstruct the 

 white light, casts a red shadow. The stoppage of the 

 white light permits the redness to reappear. This might 

 be used in melodrama with powerful stage effect. A 

 black figure could be made to cast a blood-red shadow as 

 he passed a white-looking wall. The experiment in a room 

 with the outspread fingers is very striking. Many similar 

 arrangements will occur to your readers, and among them 

 not the least interesting is the production of sulijective 

 tints by throwing with a lantern on a screen the images 

 obtained by Wheatstone's superposed discs of perforated 

 •■•■-- It is easy to fit in the ordinary wooden slide-holder 



and to fix to it a circle of similar zinc, so as to rotate 

 freely on a pin. The circle should be wide enough to pro- 

 ject a little above the frame, and be eatily reached with 

 a finger, to make it revolve. Colouitd gelatine films 

 lilacecl close to thi' front lens of the lantern will give the 

 effects required. 



"THE GREEN-BEAM PAPER." 



(A Sequel.) 



BV LlKLT.-C0I,0.\F.L W. A. Ross, LATE E.A. 



A SUNNY day — "happy Saturday," April 8 — which has inter- 

 vened since the publicatioD in Knowleuge (page 496), of 

 the article, aptly named by you as above, having enabled me to add a 

 few more auxiliary experiments to the original one there detailed, 

 made on tlio 7th of October last, I would ask of your courtesy a 

 little space in an early number of our delightful " Mag." for a 

 brief description of them before your readers forget my former 

 paper. 



1. A plate of blue-violet glass, 01G2 inch thick (the thickest I 

 had), transmitting blue-violet light, but from which light, so trans- 

 mitted, a sheet of white paper reflected reildi.ih-violet light, wiig 

 held in the path of a sunbeam admitted thn)«;,'h the open window 

 of a scarcely-darkened room, and condensed by a lens about 2 inched 

 in diameter, in two positions — («) so that the sunbeam was focussed 

 0)1 the glass ; (h) so that the sunbeam was focussed on the paper 

 about half a foot beneath it, Ihroujh the glass. In the case of (a) 

 the solar image on the paper was large of course, and oval, but of a 

 pure blue colour ; being apparently the result of the elimination of 

 red rays by the thick glass, and of yellow rays by the blue part of 

 the blue-violet glass, leaving the blue rays only for transmission. 

 In the case of (b) the focussed solar image was small, nearly 

 circular, and of a pure blue colour, contrasting strongly with 

 the violet transmitted to the paper by the rest of the glass. 

 As the mixture "* red and blue lights, in certain proportion, un- 

 doubtedly forms violei. -Sght; and, as blue or bluish glass inter- 

 cepts yellow rays, this blue colour was, on this hypothesis, to 

 be expected, and it was supposed by me that the purity of the blue 

 focus on the paper was due to the thick glass rutting off red rays, 

 according to the exiieriment detailed in Knuwlkdge of April 7, and 

 to the blue in the glass cutting off yellow rays, leaving only pure 

 blue rays to pass on to the paper. 



The focus (a) appeared at tirst of a pale orange colour on the 

 surface of the glass, but soon became greenish, perhaps from the 

 simultaneous transmission of some reflected blue rays given out by 

 the main body of the glass to the retina. 



2. A similarly tinted violet glass, but only half the thickness of 

 1, was held, with reference to the lens and paper, in the positions 

 1 (a) and 1 (h) ; when — (n) there was a large violet image on the 

 paper, but with a distinct reddish border. 



(6) The focussed solar image was small, and of a faint bluish-violet 

 colour, so extremely pale as to be nearly white, and tolerably 

 brilliant as reflected directly from the paper ; but this bluish-white 

 brilliant image, viewed through the glass itself, appeared of a pale 

 violet colour, with a bright pure red or " crimson " border. Xo red 

 was observable tlirough the glass of 1 (h). 



3. After dark, a thick platinum wire, bent at a right angle, and 

 heated to faint redness before a blowpipe, was viewed through 

 glass 2, when the " blue" blowpipe pyrocone appeared a rcil violet, 

 and the wire a brilliant red colour, similar to the border 2 (6), 

 which became bluish white when the wire was made white hot. 



Am I wrong, then, in supposing that the thin glass 2 did not 

 effectually cut off the red rays, some of which, therefore, escaped 

 through it from the lens to the nearly white focus on the paper, 

 and were contained in the reflection of that which passed directly 

 to the retina, although invisible there ; but when the violet glass 

 intervened, the red rays were seen to be )iartially separated from 

 the blue rays in their vibrations through the glass, and jdaced as 

 the outer border of the pale-violet focus r* 



■1. A green-blue glass, the thickness of 2, was treated with sun- 

 beam and lens similarly to 1 and 2, but the only remarkable phe- 

 nonienon I could observe was that the rays reflected from the focus 

 on the glass in position a were an almost brilliant rcddisli orange 

 colour. 



5. A slip of thin ordinary glass (a microscope slide) transmitted 

 the focussed sunbeam as white light through its shorter axis, but 

 when turned edgeways, the " green beam " immediately appeared, 

 and it could even thus, by turning the glass slowly round, be ascer- 



,....v. . . ., ^„,y ^ Ml, u. t.,c orinnary wootien Slitle-nolder tained to what depth the beam could travel as white light : so that 

 a square 01 the /inc, winch may be fastened to the frame, | it would appear as if the red rays were absorbed, and not "cut oft" 



