Mat 19, 1882.] 



• KNO^AALEDGE • 



603 



photographs of clouds, and no opportunity should be lost 

 to obtain negatives of various size, or they may be taken 

 <in small plates, and aftenvards enlarged. These negatives 

 ;iro to be used for printing in the skies. Always be careful 

 to select a cloud negative with the light falling in the same 

 direction as in the picture. Place the print on a Hat board 

 0.- slieet of glass, and then put over it the cloud negative, 

 c.irefidly arranging it so that tlic masses of cloud fall 

 suitably. Cover the lower part of the print with a piece 

 of cardboard, so that no light can injure it; but the top 

 part of the cardVioard must be bent ujnvards, permitting 

 the light to graduate towards the horizon of the print 

 ■ Now expose to difluse daylight to print the clouds. This 

 will very soon be done, as tlie printing must not be carried 

 too far, or the results will be heavy and unplcasing. 



Success will depend very much on care and cleanliness 

 in all the various operations described. The dish used for 

 nitrate of silver solution should not be used for any other 

 purpose. All bottles should be carefully labelled, and 

 always put in proper places. Glass measures should .always 

 be washed out after use. The bottles containing collodion, 

 developing, fixing, and other solutions, should always be 

 kept in the same places wliile in use, and preferably should 

 be of different shapes, so as to avoid mistakes in the feeble 

 light of the darkened room. The glass vessel, or bath, 

 containing nitrate of silver solution, should liave a wooden 

 case made to slope at a suitable angle. This case is to 

 protect the coUodionised plate from light caused by the 

 chance opening of a door, and if the top be protected by a 

 suitable cover, the plate may be safely left with a flood of 

 ■white light in the room while the operator proceeds with 

 other work. The dark slide of the camera should be wiped 

 drj- after using each plate. Thick blotting-paper may be 

 nsed for this purpose. Nitrate of silver stains on linen 

 are troublesome to remove, therefore keep certain cloths 

 for certain purposes. To remove accidental stains, a strong 

 solution of cyanide of potassium should Vie used, and the 

 parts should then be washed with soap and water. Stains 

 on the hands may be removed by rubbing them with solu- 

 tion of iodine and then witli cyanide of potassium. Dilute 

 hydrochloric acid will also remove stains from the skin. 



CURIOSITIES OF COLOUR. 



By Hexkv J. Slack, F.G.S., F.R.M.S. 



INTENDING to resume the " Studies of Minute Life " 

 by a notice of the micro-ferments and M. Pasteur's 

 discoveries, the writer may be permitted to indulge in a 

 temporary change of subject, and mention some interesting 

 experiments with colour, in the hope of thus supplying 

 answers to some of tlie questions tliat have been put by 

 readers of Knowledoe. 



A couple of prisms will enable two spectra of the sun 

 to be thrown upon a white ceiling, or sheet of wliite paper 

 fastened to the wall. By a little management, the two 

 spectra may be wholly or partially superposed, and the 

 very different behaviour of light rays from pigments thus 

 displayed. The exact and coincident superposition of two 

 similar spectra will reinforce the colours, and if one is 

 thus made to slide over the other, the effects of combina- 

 tion will by no means agree with the mixtures of analogous 

 pigments. The red and the green will give a yellow, and 

 this result of combining red and green is also made evident 

 by Lord Rayleigh's " New Colour Combination Plates," 

 made by Browning. In this apparatus a blue disc is super- 

 posed over a yellow one, and then, as tlie inventor says, 

 " the yellow disc absorbs the blue light The blue disc 



absorbs the yellow and orange light By superposing the 

 discs, and viewing a white object, as a cloud, the 

 resultant light will be yellow. But on analysing the 

 same with a spectroscope, it is proved that yellow liglit is 

 produced by a commingling of red and green, since these 

 rays of the spectrum arc alone transmitted." 



The success of the experiment depends upon hitting the 

 exact tints of the two colours. In the specimen before the 

 writer, the yellow is rather orangey, and when examined 

 with the spectroscope shows colours from deep red to a 

 blue-green. The blue disc analysed in th(! same way shows 

 a band of bright red, succeeded by a cloudy one, something 

 like a dingy, brownish puce, and then green, blue, and 

 violet ; no yellow. The combination, which gives an 

 orangey yellow, when analysed shows red and green, and a 

 cloudy band in place of the orange and yellow. The blue 

 and violet are suppressed. 



Another way of making interesting experiments is by 

 obtaining from an artists' colour-shop (such as Brodie 

 it Middleton, in Long-acre) various sheets of coloured 

 gelatine and a sheet of black cardboard. Cut some of 

 the cardboard into 4-inch squares ; make a round hole 

 in the middle of each square, l.i inches in diameter. Over 

 these holes fasten coloured gelatine of all the chief obtain- 

 able tints. This is done most usually by laying the 

 gelatine on the unblackened side of the cardboard, and 

 gumming over it a square paper with a hole in it corre- 

 sponding with that in the cardboard. This keeps the 

 galatine in its place. When one of the squares so prepared 

 is held near the eye, the black part e.xcludes most of the 

 difl'used white light, and objects in a garden or in a room 

 may be seen as affected by the tint of the gelatine, with a 

 result that is often very different from what might be 

 expected. For example, a sky-blue disc slightly blues 

 white flowers, gives a dingy purplish hue to the rich pink 

 of dielytra, and makes the yellow-green of aucuba leaves less 

 noticeable. After this take a disc of purple, like the 

 common small blue glass so often misapplied in cockney 

 conservatories. Many green leaves now flash out with red 

 tints. At this moment the writer sees this effect in young 

 April rose leaves, the lightest aucuba leaves, the golden 

 green of Tloija aiirea, and many more. Polyanthus and 

 winter aconite leaves assume a dull lurid tint, while the 

 dielytra blossoms have a richer glow. A crimson disc 

 quite changes the bright leaves of aucuba and Thuja aurea. 

 This shrub looks as if covered with orange-yellow flowers, 

 while the sky, now covered with a pale cloud, takes the 

 full crimson hue. 



All the olyects at the time of writing are in a fairly 

 strong white daylight, and the leaves, according to their 

 character and the angle under which they are seen, reflect 

 more or less white light, besides exerting their normal 

 effects of absorbing portions of the spectrum. 



For another set of experiments, cut some pieces of black 

 cardboard six inches long and four wide ; punch, or cut, 

 then, half-inch in diameter round holes. Cover one set 

 severally with a blue, a crimson red, and a yellow piece of 

 gelatine. Hold it up against a white cloud ; look at it 

 with a double image prism. By keeping the prism near 

 the eye, and holding the cardboard at a suitable distance, 

 to be found by experiment, and then rotating the prism, 

 it is easy to make any one of the coloured discs overlap 

 any other one. Sky-blue and pale yellow thus treated 

 give nearly white light. If the same tints of pigments 

 were mingled, the result would be a pale green. The yellow 

 superposed upon the crimson makes it orange. Super- 

 po.sing the pale blue upon the crimson gives a whitish, 

 violetish grey, the tint varying with the quantity of white 

 light reflected from the cloud. 



