8

SELECTION OF TAPER FOR PHOTOGRAPHS.

would be produced by free exposure during three or four times the giventime. When fixed by hyposulphite of soda, the difference is renderedyet more striking, to an extent hardly credible without trial.” It wasalso found that “ the same glass in several instances exercised quite asremarkable an influence in depressing, as in others it did in exalting thesolar action.” The researches of the author of the present volume, allgo to prove that this peculiar effect is mainly dependent on the qualityof the paper employed. It is certain that the result is considerablyinfluenced by it This being the case, the necessity of endeavouring toestablish by experiment, some data, by which we might always throwthe balance of action on the exalting side, is very evident, as by thismeans we are enabled to increase the strength and decision of outline inan extraordinary degree. The following tables will exhibit the resultsof an extensive series of experiments, undertaken after the publicationof Sir J. Ilerschel’s memoir “ on the Chemical Action of the Rays ofthe Solar Spectrum,” in which he has given a table of results, obtainedwith different preparations on various kinds of paper; but as he has notestablished the influence of the paper, except in a few instances, in-dependent of the preparation, it became desirable to endeavour to do so.

In pursuing this inquiry, it was found that the same description ofpaper, from different manufacturers, gave rise to widely different effects;so that the most carefully conducted experiments, several times repeated,have only given approximations to the truth; and it is advised thateach inquirer should himself try the effect of plate glass, in exalting ordepressing the darkening property of the silver, on the paper he mayemploy. In the event of this being noglocted, the following table willbe of considerable utility;—

I.-PATERS PREPARED WITH MURIATE OF SODA.

a. Superfine satin post,

b. Thick wove post,.

c. Superfine demy.

d. Bath drawing card,..

e. Thick post,..

f Common bank post,..

g. Thin post,.

h. Tissue paper,.

Considerable exalting effect.Depressing influence.

Slight exalting effect.

No influence.

Slight exalting effect.

Ditto.

No influence.

Considerable exalting effect.

II.— TAPERS PREPARED WITH MURIATE OF BARYTES.

a. Superfine satin post.Slight exalting influence.

b. Thick wove post,. Ditto, but stronger.

c. Superfine demy,.Similar to a.

d. Bath drawing card,.Similar to a.

e. Thick post.Considerable exalting influence.

f Common bank post,.Similar to a.

g. Thin post.Similar to e.

h. Tissue paper,.i.Results uncertain.