I find that the pictures on a French stereoscopic slide I have by me have been taken at an angle of 10 deg., or 1 in 6. This was evidently photographed at a considerable distance, the triumphal arch in the Place de Carousel (of which it is a representation) being reduced to about 11/4 inch in height. How comes it then that the angle is here increased to 10 deg. from 61/2 deg., or to 1 in 6 from 1 in 9. Moreover, the only work I have been able to obtain on the mode of taking stereoscopic pictures, lays it down that all portraits, or near objects, should be taken under an angle of 15 deg., or, as it says, 1 in 5; that is, if the camera is 20 feet from the sitter, the distance between its first and second position (supposing only one to be used) should not exceed 4 feet: otherwise, adds the author, "the stereosity will appear unnaturally great." When two cameras are employed, the instructions proceed to state that the distance between them would be about 1/10th of the distance from the part of the object focussed. The example given is a group of portraits, and the angle, 1 in 10, is afterwards spoken of as being equivalent to an arc of 10 deg. Farther on, we are told that "the angle should be lessened as the distance between the nearest and farthest objects increase. Example: if the farthest object be twice as far from the camera as the near object, the angle should be 5 deg. to a central point between these two. Now, I find by calculation that the measurements and the angle here mentioned by no means {506} agree. For instance, an angle of 15 deg. is spoken of as being equivalent to the measurement 1 in 5. An angle of 10 deg. is said, or implied, to be the same as 1 in 10. This is far from being the fact. According to my calculations, the following are the real equivalents:-- An angle of 15 deg. is equal to 1 in 4. " 12 deg. " 1 in 5. " 10 deg. " 1 in 6. " 6-1/2 deg. " 1 in 9. " 6 deg. " 1 in 10. " 5 deg. " 1 in 12. " 4 deg. " 1 in 15. Will any of your readers oblige me by solving the above anomalies, and by giving the proper angles or measurement under which objects should be taken when near, moderately distant, or far removed from the camera; stating, at the same time, at how many feet from the camera an object is to be considered as near, or distant, or between the two? It would be a great assistance to beginners in the stereoscopic art, if some experienced gentleman would state the best distances and angles for taking busts, portraits, groups, buildings, and landscapes. It is said that stereoscopic pictures at great distances, such as views, should be taken "with a small aperture." But as the exact dimensions are not mentioned, it would be equally serviceable if, to the other details, were added some account of the dimensions of the apertures required for the several angles.