290 THE HISTORY OF SOME DISCOVERIES OF PHOTOGRAPHY. 



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

 and, after l^eing 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 obtained. These 

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

 makes the following remarks on the subject: 



Tlio substiince wliioh should be used in preference to bitumen is the residimni 

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

 alcohol and n])])lied in an extremely thin wash. Although all bitnminons and 

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

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

 which are the most luictuous, becaxise 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, liy 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 nnich more liable to injui-y 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 by 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 should be transparent and of a lemon-yellow color. The clearer 

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

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

 coat, indispensable re(iuisites for obtaining good effects in photographic designs. 

 The use of the alcohol or ether is to facilitate the ai)plicati()n of the 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 about sketches taken 

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



Before commencing operations the experimenter nuist carefully clean his 

 glass or metal plate. For this jmrpose emery reduced to an impali)nhl(> jiowder 

 mixed with alcohol may l)e used, applying it by means of cotton wool, but this 

 part (»f the process nmst always be concluded by dry polishing, that no trace of 

 moisture may remain on the tablet. The plate of mettil or glass being thus pre- 

 pared, in order to ^•npply the wash or coating it is held in one hand and with 

 the other the solution is to be poured over it from a flask or bottle having a 

 wide mouth, so tliat it may flow rajiidly and cover the whole surface, li is at 

 first necessary to bold the plate a little inclined, but as soon as the solution 

 is poured on and has ceased to flow freely it is raised perpendicularly. The 

 finger is then p.-issed behind and below the i)late in order to draw off a i)ortion 

 of tlie li(|uid, which, tending always to ascend, woidd double the thicknes^s of 

 the covering. Tlie finger nuist be wiped i>ach time, and l)e passed very rapidly 



