T 



5S6 



KNOWLEDGE 



[May 12, 1882, 



when tlie photograph is to be coloured. Uoth kinds may 

 be purcliased rcudy for use, excepting that it is not sensi- 

 tised.* The necessary appliances, such as printing-frames, 

 porcelain dishes, and other articles, which will be named 

 in due course, it is assumed will have been supplied in the 

 "outHt." We now require a solution of nitrate of silver, 

 and this must be prepared in the proportion of 40 or 50 

 grains to each ounce of water. The quantity required 

 will depend on the size of the pieces of paper to be pre- 

 pared — say a quarter of a sheet ; the dish, therefore, must 

 be somewhat larger, and solution of silver must be poured 

 in to the depth of about lialf-an-inch ; then take the paper 

 by two corners, and place it on the solution so that it will 

 fall down evenly. After a few moments, with a piece of 

 horn or wood, lift the paper by one corner, so that it may 

 be seen that there are no air-bubbles, which, if any are 

 present, may be removed by a puff of breath, or they may 

 be touched. The paper is to bo then replaced, and left for 

 about three minutes — not longer. It may then be lifted 

 slowly, and, after draining the surplus solution (this may 

 be facilitated by drawing the paper over the edge of the 

 porcelain dish) the sheet may be suspended to a line by 

 American " clips," or it may be dried at once by fire-heat. 

 Cut the paper, which must be quite dry, to the size 

 required. Place the negative face, or picture side, upwards 

 in the pressure-frame ; put the paper carefully on the 

 negative, then put on the back of the frame, the paper 

 being pressed close by springs or screws, care being taken 

 not to apply more pressure thrm is necessary to keep the 

 paper and negative in perfect contact. The frame is then 

 to be placed in strong daylight ; diffused light is preferable 

 to the direct rays of the sun. 



While we suppose the printing is proceeding, we may 

 say that the strength of the silver solution should not be 

 allowed to fall much below 40 grains to the ounce, other- 

 wise the prints will be weak. A good plan is to keep a 

 stock of full strength always ready, and always fill up the 

 bottle of solution to replace the portion used. If it should 

 be found that the silver solution becomes discoloured by 

 use, the colour may be removed by the addition of kaolin 

 or China clay ; shake well after each time of using the bath, 

 and when the kaolin has subsided, the silver solution will 

 be found to be quite clear, and may be poured off for \ise 

 when required. Filtration is sometimes desirable. If 

 the surface of the silver has any scum upon it, this is a 

 sure sign that the bath should be filtered, but the scum 

 can sometimes be removed by drawing the edge of a piece 

 of blotting-paper over the solution. 



The strength of the solution of silver may be determined 

 with sufficient accuracy by means ofthe argentometer sold 

 for the purpose. 



The progress of the print must be watched. It will be 

 noticed that the paper projecting beyond the edges of the 

 negative has changed to a dark tint. If the negative be a 

 dense one, this darkening may be allowed to proceed until 

 the paper is nearly black. The frame should now be re- 

 moved out of the full daylight, and one-half the back of 

 the frame opened, when, on turning back the print, it will 

 be seen whether the printing is deep enough, and it re- 

 quires some little experience to determine this. The print 

 should be rather over printed, as the fixing will reduce it to 

 some extent If not printed deep enough, the finished 

 print will appear weak and unsatisfactory. When as many 

 prints as are required are ready, they should be put, one at 

 a time, into water, in order to remove the excess of chloride 

 of silver ; after a few minutes' washing, the water may be 



• If preferred, paper may bo purclmsod ready sensitised, but we 

 aasamo the amateur will wish to prepare his own. 



poured into a jar for the purpose of recovering the silver, 

 aiid then, after one or two changes of water (if the second 

 washing water appears milky, it still contains silver), the 

 prints are ready for the next operation — toning. :i 



If the prints were to he fixed without toning, they would 

 not have a pleasing colour. We now require another 

 solution, which contains chloride of gold. Tliis salt is 

 supplied in small glass tubes containing 15 grains. The 

 tube may be broken and thrown into a bottle containing 

 15 ounces of water; there will therefore be one grain of 

 gold to each ounce of water. The use of this solution of 

 gold is for the purpose of changing the colour of the print ; 

 the change produced being a combination between the gold 

 and silver in the paper, the gold giving a better colour to 

 the finished print, and the gold-toned print is also more 

 permanent. 



There are many kinds of toning-baths, but the one which 

 gives good results, is easy to make and use (it may be used 

 as soon as made) ; it consists of one ounce of the solution of 

 gold, and, say, 20 ounces of water poured into a porcelain 

 dish. A small piece of litmus paper put into the solutioa 

 will show that it is acid. A lump of carbonate of soda (com- 

 mon washing soda) may be put into the solution, and 

 stirred about until the litmus paper shows that the solu- 

 tion is slightly alkaline. The prints may now be put into 

 this alkaline gold solution, face downwards, a few at a 

 time, and they must be kept in constant motion, to insure 

 equal toning. As soon as the prints present a pleasing 

 colour, they may be removed into a dish of clean water, 

 and when all are toned, they are ready for the next opera- 

 tion — fixing. It may be remarked here that one grain of 

 gold will be sufficient to tone a sL^-^*^ of paper ; therefore 

 the proportion to be iised for each batcli^of prints must be 

 regulated accordingly. 



Iha fixing is efl'ected by means of hyposulphite of soda, 

 in the proportion of 3 ounces of the salt to 20 ounces of 

 water. The prints must be left in the soda solution about 

 20 minutes, and they must be kept in motion for a time, 

 to insure even action in fixing. When fixed, the prints 

 must be removed to a dish of water, and the water must be 

 frequently changed during the first half-hour. The prints 

 may now be left during the night in the water, and the 

 following day, after several changes, they may be placed 

 between clean cloths or blotting-paper, and if they have 

 been previously trimmed, they are ready for mounting. 



THE COMING TRANSIT OF VENUS. 



By R. A. Proctor. 



I PROPOSE to give a short and simple account in these 

 pages, in the next few weeks, of the circumstances 

 under which transits of Yenus occur, explaining the 

 general principles on which the determination of the sun's 

 distance by observation of Yenus in transit depends. But, 

 as many questions have been addressed to me respecting 

 the places where the whole transit — its beginning and its 

 end — will be seen, it appears well to give at the outset, for 

 immediate reference, a chart showing from what parts of 

 the earth the transit can be seen wholly or in part. 



The following preliminary explanation may be useful, 

 however : — • 



Yenus circuiting around the sun in a smaller orbit than 

 the earth, and completing a circuit in 224 w days, whereas 

 the earth takes 365] days, passes between the earth and 

 sun at intervals of about 583 9 days, or a year and seven 

 months. If she travelled in the same plane as the earth, 

 she could not thus pass between the earth and the sun 



