Handbook of American Daguerrotype | Page 8

moisture may arise as the result of the cold, from a want of friction in the buffing of the plate, which, coming in contact with the warmer air, as a writer on this subject says:
"It is well known that as often as bodies, when cold, are exposed to a warmer air, the humidity contained in them is condensed. It is to this effect that we must attribute the difficulty experienced in operating in most cases." This is corroborated by the results experienced by our operators. So it is seen that the plate should be of a temperature above that of the atmosphere. Mr. Gurney submits his plates to a gentle heat from a spirit lamp just before exposing them to the vapor of iodine. Experience has convinced me that a plate heated to about 80 deg. before being exposed to iodine will present a far better defined image than aplate at a temperature of 50 deg. I account for this by noticing that, at a higher temperature, the plate throws off any larger crystals that might otherwise be deposited, receiving only the finer, thus producing a more perfect chemical combination of iodide of silver. I would call the attention of the operator to this point, as presenting something of interest, and which may direct in a way of accelerating the future operations.
That the presence of a film of moisture over the plate is a preventive of uniform chemical action, may be readily understood from the fact that iodine is almost insoluble in water, requiring seven thousand parts of water to dissolve one of iodine, or one grain to a gallon of water. Yet its affinities for silver and other substances are so powerful as to prevent its existing in an insulated state, hence we can account for the frequent occurrence of a plate presenting parts of an image over its surface. It is quite evident that those parts of plate's surface covered with moisture are nothing like as sensitive to the iodine as those parts perfectly free.
Exposure of the plate in the Camera, and Position.--The time of exposure necessary to produce an image upon the Daguerreotype plate, can only be determined by experiment, and requires a liberality of judgment to be exercised on the part of the operator. The constant variation of the light renders it impossible to lay down any exact rule upon this point. Light is not alone to be considered; the amount of coating exercises a deviating influence, also the subjects to be represented are not equally photogenic, some requiring much longer time of exposure than others. This may be easily observed by exposing the plate at the same time to a plaster bust and a piece of black velvet, the first being a much stronger reflector of light than the latter: the time necessary to produce a well developed image of the velvet being about six times longer than that required to produce an equally defined image of plaster. The manner of judging correctly of the time is by the appearance of impression after it has been developed by the mercurial vapors. Should it present a deep blue or black appearance it is solarized or over-timed. This sometimes is to an extent, that a perfect negative is formed, the white being represented black, and the dark light.
An object requiring the particular care and attention of the operator is the proper focus. It is not unfrequently the complaint of sitters that their hands are represented as being magnified and greatly out of proportion with the general figure. This is the case also with the nose and eyes, but in a less degree. As this cannot be wholly remedied, it is desirous to come as near as possible, and in order to do this, it is necessary to present the figure in such a position as to bring it as nearly as possible upon the same plane by making all parts nearly at equal distance from the lenses. This must be done by the sitter inclining the head and bust formed to a natural, easy position, and placing the hands closely to the body, thus preserving a propel proportion, and giving a lively familiarity to the general impression. It is not an uncommon fault among our less experienced operators to give a front view of the face of nearly every individual, regardless of any particular form, and this is often insisted upon by the sitter,* who seems to think the truth of the picture exists principally in the eyes staring the beholder full in the face.
* I might here picture some curious scenes experienced by our operators Every one is familiar with a certain class of our community whose ideas of the importance of a free and easy position of the body are too closely confined with stays,