It is a painful and ungracious task to have to pen these observations, especially, too, in the case of a stranger. But "N. & Q." must not be made a channel for erroneous statements, and we "natives and to the manner born" must be allowed to know best what is in our own museums. W. PINKERTON. Ham. * * * * * PHOTOGRAPHIC CORRESPONDENCE. _Stereoscopic Angles._--Like many of your correspondents, I have been an inquirer on the subject of stereoscopic angles, which seems to be still a problem for solution. What is this problem? for until that be known, we cannot hope for a solution. I would ask, is it this?--_Stereoscopic pictures should create in the mind precisely such a conception as the two eyes would if viewing the object represented by the stereograph._ If this be the problem (and I cannot conceive otherwise), its solution is simple enough, as it consists in placing the cameras _invariably_ 2-1/2 inches apart, on a line parallel to the building, or a plane passing through such a figure as a statue, &c. In this mode of treatment we should have two pictures possessing like stereosity with those on the retinas, and consequently with like result and as our eyes enable us to conceive perfectly of any solid figure, so would the stereograph. I believe, therefore, that this is, under every circumstance, the correct treatment; simply because every other mode may be proved to be false to nature. Professor Wheatstone recommends 1 in 25 when objects are more than 50 feet distant, and this rule seems to be pretty generally followed. Its incorrectness admits of easy demonstration. Suppose a wall 300 feet in extent, with abutments, each two feet in front, and projecting two feet from the wall, at intervals of five feet. The proper distance from the observer ought to be 450 feet, which, agreeably with this rule, would require a space of 18 feet between the cameras. Under this treatment the result would be, that both of the _sides, as well as the fronts_, of the three central abutments would be seen; whilst of all the rest, only the front and one side would be visible. This would be outraging nature, and false, and therefore should, I believe, be rejected. The eyes of an observer situated midway between the cameras, could not possibly perceive either of the sides of the buttress opposite to him, and only the side next to him of the rest. This seems to me conclusive.