290 THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 



tioii is of much greater tenacity and higher color than the former, 

 and, after being applied, it can immediately be submitted to the 

 action of the light, which appears to render it solid more quickly, 

 from the greater volatility of the animal oil. M. Daguerre remarks 

 that this very properly diminishes still further the resources of the 

 process as respects the lights of the drawings thus ol^tained. These 

 processes of M. Niepce Avere much improved by M. Daguerre, who 

 makes the following remarks on the subject: 



The substance wliich should be used in preference to bitumen is tlie residuum 

 obtained by evaporating the essential oil of lavender, which is to be dissolved in 

 alcohol and applied in an extremely thin wash. Although all bituminous and 

 resinous substances are, without any exception, endowed with the same prop- 

 erty — that of being affected by light — the preference ought to be given to tliose 

 which are the most unctuous, because they give greater firmness to the draw- 

 ings. Several essential oils lose this character when they are exposed to too 

 strong a heat. 



It is not, however, from the ease with which it is decomposed that we are to 

 prefer the essential oil of lavender. There are, for instance, the resins, which, 

 being dissolved in alcohol and spread upon glass or metal, leave, by the evapo- 

 ration of the spirit, a very white and infinitely sensitive coating. But this 

 greater sensibility to light, caused by a quicker oxidation, renders also the 

 images obtained nmch more liable to injury from the agent by which they were 

 created. They grow faint and disappear altogether when exposed but for a few 

 months to the sun. The residuum of the essential oil of lavender is more 

 effectually fixed, but even this is not altogether uninfluenced l)y the eroding 

 effects of a direct exposure to the sun's light. 



The essence is evaporated in a shallow dish by heat till the resinous residuum 

 acquires such a consistency that when cold it rings on being struck with the 

 point of a knife, and flies off in pieces when separated from the dish. A small 

 quantity of this material is afterwards to be dissolved in alcohol or ether ; the 

 solution formed sliould be transparent and of a lemon-yellow color. The clearer 

 the solution the more delicate will be the coating on the plate. It nuist not, 

 however, be too thin, because it would not thicken or spread out into a white 

 coat, indispensable requisites for obtaining good effects in photographic designs. 

 The use of the alcohol or ether is to faeilitate the application of tlie resin under 

 a very attenuated form, the spirit being entirely evaporated before the light 

 effects its delineations on the tablet. In order to obtain greater vigor the metal 

 ought to have an exquisite polish. There is more charm altout sketches taken 

 on glass plates, and, above all, much greater delicacy. 



I'efore conuuencing operations the experimenter must carefully clean his 

 glass or metal plate. P"'or this purpose emery reduced to an inii)alpal)le ])o\vder 

 mixed with alcohol may be used, ai)plying it by means of cotton wool, but this 

 part of the process nuist always be concluded by dry polisliing, that no trace of 

 moisture may reinain on the tablet. Tlie plate of metal or glass l)eiug thus pre- 

 pared, in order to supply the wash or coating it is licld in one hand and with 

 llic otlier tlie solution is to be i)oured over it from a flasU or liottle liaving a 

 wide mouth, so that it may flow rai)idly and cover tlie whole surface. It is at 

 lirst necessary to hold the plate a little iix-lined, but as soon as th(> solution 

 is poured on and has ceased to flow freely it is raised i)erpendicularly. Tlie 

 finger is then passed l)ehind and b<'low tlie i)late in order to draw off a jiortion 

 of the li(|uid, wliicli, tending always to .ascend, would double the thickness of 

 the covering. Tlie finger must be wiped each time, and be passed very ra])idly 



