[In the construction of a photographic house, we beg to inform our correspondent that it is by no means needful to use entirely violet-coloured glass, but the roof thereof exposed to the rays of the sun should be so protected; for although the light is much subdued, and the glare so painful to the eyes of the sitter is taken away, yet but few of the actinic rays are obstructed. It has been proposed to coat the interior with smalt mixed with starch, and afterwards varnished; but this does not appear to have answered. Calico, both white and coloured, has also been used, but it is certainly not so effectual or pleasant. Upon the whole, we think that the main things to attend to are, firmness in its construction, so as to avoid vibration; ample size, so as to allow not only of room for the operator, but also for the arrangements of background, &c., and the sides to open so as to allow a free circulation of air; blinds to be _applied at such spots only_ as shall be found requisite. Adjoining, or in one corner, a small closet should be provided, admitting only yellow light, which may be effectually accomplished by means of yellow calico. A free supply of water is indispensable, which may be conveyed both to and from by means of the gutta percha tubing now in such general use. We apprehend, however, that the old proverb, "You must cut your coat according to your cloth," is most especially applicable to our querist, for not only must the house be constructed according to the advantages afforded by the locality, but the amount of expense will be very differently thought of by different persons: one will be content with any moderate arrangement which will answer the purpose, where another will be scarcely satisfied unless everything is quite of an _orne_ character.] _Dr. Diamond's Replies._--I am sorry I have not before replied to the Queries of your correspondent W. F. E., contained in Vol. viii., p. 41.; but absence from home, together with a pressure of public duties here, has prevented me from so doing. 1st. No doubt a _small_ portion of nitrate of potash is formed when the iodized collodion is immersed in the bath of nitrate of silver, by mutual decomposition; but it is in so small a quantity as not to deteriorate the bath.