In the directions given in the work from which I have quoted, it is said that when pictures are taken with one camera placed in different positions, the angle should be 15 deg.; but when taken with two cameras, the angle should be 10 deg. Is this right? And, if so, why the difference? In the account given by you of Mr. Wilkinson's ingenious mode of levelling the cameras for stereoscopic pictures, it is said the plumb-line should be three feet long, and that the diagonal lines drawn on the ground glass should be made to cut the principal object focussed on the glass; and "when you have moved it, the camera, 8 _or_ 10 feet, make it cut the same object again." At what distance is the object presumed to be? Any information upon the above matters will be a great service, and consequently no slight favour conferred upon your constant reader since the photographic correspondence has been commenced. [phi]. _Photographic Portraits of Criminals, &c._--Such experience as I have had both in drawing portraits and taking photographs, impels me to hint to the authorities of Scotland Yard that they will by no means find taking the portraits of gentlemen that are "wanted" infallible, and I anticipate some unpleasant mistakes will ere long arise. I have observed that inability to recognize a portrait is as frequent in the case of photographs as on canvass, or in any other way. I defy the whole world of artists to reduce the why and wherefore into a reasonable shape; one will declare that "either" looks as if the individual was going to cry; the next critic will say he sees nothing but a pleasant smile. "I should never have known who it is if you hadn't told me," says a third; the next says "it's his eyes, but not his nose;" and perhaps the next will say, "it's his nose, but not his eyes." I was present not long since at the showing a portrait, which I think about the climax of doubt. "Not a bit like," was the first exclamation. The poor artist sank into his chair; after, however, a brief contemplation, "It's very like, _in-deed_; it's excellent:" this was said by a gentleman of the highest attainments, and one of the best poets of the day. Some persons (I beg pardon of the ladies) take the habiliments as the standard of recognition. I do not accuse them of doing it wilfully; they do not know it themselves. For example, Miss Smith will know Miss Jones a mile or so off. By her general air, or her face? Oh no! It's by the bonnet she helped her to choose at Madame What-d'ye-call's, because the colour suited he complexion.