_A Prize for the best Collodion._--Your "Hint to the Photographic Society" (Feb. 25) I much approve of, but I have always found more promptness from individuals than from associated bodies; and all photographers I deem to be under great obligations to _you_ in affording us a medium of communication before a Photographic Society was in existence. During the past month your valuable articles, from some of our most esteemed photographists, show that your pages are the agreeable medium of publishing their researches. I would therefore respectfully suggest that you should yourself offer a prize for the best mode of making a good useful collodion, and that that prize should be a complete set of your valuable journal, which now, I believe, is progressing with its ninth volume. You might associate two independent names with your own, in testing the merits of any sample supplied to you, and a condition should be that the formula should be published in "N. & Q." Your observations upon the manufacturers of paper, respecting the intrinsic value of a premium, are equally applicable to this proposition, because, should the collodion prepared by any of the various dealers who at present advertise in your columns be deemed to be the most satisfactory, your sanction and that of your friends alone would be an ample recompense. I would also suggest that samples sent to you should be labelled with a motto, and a corresponding motto, _sealed_, should contain the name and address, the name and address of the successful sample _alone_ to be opened: this would effectually preclude all preconceived notions entertained by the testing manipulators who are to decide on the merits of what is submitted to them. A READER OF "N. & Q." AND A PHOTOGRAPHER. [We are obliged to our correspondent not only for the compliment he has paid to our services to photography, but also for his suggestion. There are many reasons, and some sufficiently obvious, why _we_ should not undertake the task proposed; and there are as obvious reasons why it should be undertaken by the Photographic Society. That body has not only the means of securing the best judges of such matters, but an invitation from such a body would probably call into the field of competition all the best photographers, whether professional or amateur.] _Double Iodide of Silver and Potassium._--I shall feel greatly indebted to you, or to any correspondent of "N. & Q.," for information as to the proportion of iodide of silver to the ounce of water, to be afterwards taken up by a _saturated solution_ of iodide of potassium, and converted into the double iodide of silver and potassium.