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Instead of wax； boiled linseed oilit must be the best and most transparent kindmay be used。 The back of the negative photograph should be smeared with the oil； and then placed between sheets of bibulous paper。 When dry the paper is highly transparent。
IV。 APPLICATION OF PHOTOGENIC DRAWING。This method of photogenic drawing may be applied to useful purposes； such as the copying of paintings on glass by the light thrown through them on the prepared paper Imitations of etchings； which may be accomplished by covering a piece of glass with a thick coat of white oil paint； when dry； with the point of a needle； lines or scratches are to be made through the white lead ground； so as to lay the glass bare； then place the glass upon a piece of prepared paper； and expose it to the light。 Of course every line will be represented beneath of a black color； and thus an imitation etching will be produced。 It is also applicable to the delineation of microscopic objects； architecture； seulpture； landscapes and external nature。
A novel application of this art has been recently suggested； which would doubtless prove useful in very many instances。 By rendering the wood used for engravings sensitive to light； impressions may be at once made thereon； without the aid of the artist's pencil。 The preparation of the wood is simply as follows：Place its face or smooth side downwards； in a plate containing twenty grains of common salt dissolved in an ounce of water； here let it remain for five minutes； take it out and dry it； then place it again face downwards in another plate containing sixty grains of nitrate of silver to an ounce of water； here let it rest one minute； when taken out and dried in the dark it will be fit for use； and will become； on exposure to the light； of a fine brown color。 Should it be required more sensitive； it must be immersed in each solution a second time； for a few seconds only。 It will now be very soon effected by a very diffused light。
This process may be useful to carvers and wood engravers not only to those who cut the fine objects of artistical design； but still more to those who cut patterns and blocks for lace； muslin； calico…printing； paper hangings； etc。； as by this means the errors； expense and time of the draughtsman may be wholly saved； and in a minute or two the most elaborate picture or design； or the most complicated machinery； be delineated with the utmost truth and clearness。
CHAP。 IX。
CALOTYPE AND CHRYSOTYPE。
The materials and apparatus necessary for the Calotype process are
Two or Three Shallow Dishes； for holding distilled water； iodide； potassium； &c。the same water never being used for two different operations。
White Bibulous Paper。
Photogenic CameraFig。 9。
Pressure FrameFig 29。
Paper； of the very best qualitydirections for the choice of which have been already given。
A Screen of Yellow Glass。
Camels' or Badgers' hair Brushes：A seperate one being kept for each wash and solution； and which should be thoroughly cleansed immediately after using in distilled water。 That used for the gallo…nitrate is soon destroyed； owing to the rapid decomposition of that preparation。
A Graduated Measure。
Three or Four Flat Boards； to which the paper may be fixed with drawing pins。
A Hot Water Drying Apparatus； for drying the paper will also be found useful。
In preparing the Calotype paper； it is necessary to be extremely careful； not only to prevent the daylight from impringing upon it； but also to exclude； if possible； the strong glare of the candle or lamp。 This may be effected by using a shade of yellow glass or gauze； which must be placed around the light。 Light passing through such a medium will scarcely affect the sensitive compounds； the yellow glass intercepting the chemical rays。
Preparation of the Iodized Paper。Dissolve one hundred grains of crystalized nitrate of silver in six ounces of distilled water； and having fixed the paper to one of the boards； brush it over with a soft brush on one side only with this solution； a mark being placed on that side whereby it may be known。 When nearly dry dip it into a solution of iodide of potassium； containing five hundred grains of that salt dissolved in a pint of water。 When perfectly saturated with this solution； it should be washed in distilled water； drained and allowed to dry。 This is the first part of the process； and the paper so prepared is called iodized paper。 It should be kept in a port…folio or drawer until required： with this care it may be preserved for any length of time without spoiling or undergoing any change。
Mr。 Cundell finds a stronger solution of nitrate of silver preferable； and employs thirty grains to the ounce of distilled water： he also adds fifty grains of common salt to the iodide of potassium； which he applies to the marked side of the paper only。 This is the first process。
Preparation of the paper for the Camera。The second process consists in applying to the above a solution which has been named by Mr。 Talbot the 〃Gallo…Nitrate of Silver；〃 it is prepared in the following manner： Dissolve one hundred grains of crystalized nitrate of silver in two ounces of distilled water； to which is added two and two…third drachms of strong acetic acid。 This solution should be kept in a bottle carefully excluded from the light。 Now； make a solution of gallic acid in cold distilled water： the quantity dissolved is very small。 When it is required to take a picture； the two liquids above described should be mixed together in equal quantities； but as it speedily undergoe decomposition； and will not keep good for many minutes； only just sufficient for the time should be prepared； and that used without delay。 It is also well not to make much of the gallic acid solution； as it will not keep for more than a few days without spoiling。 A sheet of the iodized paper should be washed over with a brush with this mixed solution； care being taken that it be applied to the marked side。 This operation must be performed by candle light。 Let the paper rest half a minute； then dip it into one of the dishes of water； passing it beneath the surface several times； it is now allowed to drain； and dried by placing its marked side upwards； on the drying apparatus。 It is better not to touch the surface with bibulous paper。 It is now highly sensitive； and ready to receive the impression。 In practice it is found better and more economical not to mix the nitrate of silver and gallic acid； but only to brush the paper with the solution of the nitrate。
Mr。 Talbot has recently proposed some modifications in his method of preparing the calotype paper。 The paper is first iodized in the usual way； it is then washed over with a saturated solution of gallic acid in distilled water and dried。 Thus prepared he calls it the io…gallic paper： it will remain good for a considerable time if kept in a press or portfolio。 When required for use； it is washed with a solution of nitrate of silver （fifty grains to the ounce of distilled water）； and it is then fit for the camera。
Exposure in the Camera。The calotype paper thus prepared possesses a very high degree of sensibility when exposed to light； and we are thus provided with a mediu