TRANSACTIONS OF THE SECTIONS. Gl 



Instruments, &c. 



On producing the Idea of Distance in the Stereoscope. By Joseph Beck. 



In a view taken through the camera no immediate foreground can be introduced : 

 thus we lose in the photograph an important element in nature for the appreciation 

 of size and distance. In reproducing nature we ought to supply some substitute. 

 This can easily be accomplished. Take an ordinary glass transparent view, and look 

 carefully at it ; in some instances the foreground absolutely appears to project into 

 the instrument ; and never is it so arranged that the idea of the distance of the fore- 

 ground of the picture from the edge of the stereoscope is given. Take now a black 

 mat or card, with two holes so cut in it, that/when laid on the view, the right eye can 

 see more of the left-hand side of the right picture, and the left eye can see more of 

 the right-hand side of the left picture. It will then be obvious that the excentricity 

 of this mat will indicate a difference of angle ; and in proportion as this excentricity 

 is increased or decreased, so the picture appears to advance or recede from the stereo- 

 scope ; and as the view recedes and distance is given, so the appearance of the real 

 size of nature is obtained. 



If the plan is reversed, and the mat is cut so that the right eye sees less of the left- 

 hand side of the right picture than the left eye, we can produce the appearance of the 

 object standing up in the instrument, and in proportion as it approaches the stereo- 

 scope, so the size is decreased. In these cases there is no difference in the angle at 

 which the pictures are taken, and yet such vast differences in the apparent size of the 

 picture, showing that whilst the amount of difference of angle is a matter of compa- 

 ratively but little consequence, the introduction of a prominent foreground, such 

 as mentioned above, enables us to estimate the real size of the object viewed. The 

 carrying out of this plan may be observed in the mounting of Mr. Warren de la Rue's 

 photographs of the moon. Had they been mounted in the centre of circles, they 

 would have appeared as 2-inch balls with beautiful miniature volcanoes and mountain 

 ranges traced upon the surface ; but when mounted excentrically, they immediately 

 appear as floating far off in space, every hill and valley, mountain, volcano, or plain 

 assuming grand and imposing dimensions. 



On the Stereoscopic Angle. By A. Claudet, F.R.S. 



On the Stereomonoscope. By A. Claudet, F.R.S. 



On the Focus of Object- Glasses. By A. Claudet, F.R.S. 

 The researches on this question tended to show the relation between the distances 

 and sizes of objects with the focal distances and sizes of their images, and to find the 

 two points, one before the lens and another behind, from which the distance of ob- 

 jects and the focal distances must be measured, and from which all proportions are 

 in an exact ratio ; for it is found that measuring from the object-glass on both sides, 

 double distance of object does not produce one-half of the focal distance, and vice 

 versa. These two points are, first, the point before the lens which produces an 

 image infinitely large at infinite distance ; and behind the lens, the point which is the 

 focus for an object at infinite distance, giving an image infinitely small ; it is obvious 

 that these two points are on each side the zero of the scale of measure, and it re- 

 mained to fix trie position of another point before the lens, which produces behind 

 the lens an image as large as nature. The two spaces between these points, one in 

 front and the other behind the lens, are perfectly equal, and they are each the unit by 

 which all distances of objects and all focal distances are to be measured. Double the 

 unit in front will give a focus one-half of the unit behind the lens, and one-half of the 

 unit in front will give a focal distance double of the unit behind the lens, and all the 

 other distances in the same proportion; so that, knowing either the distance in front 

 of the lens, or the focal distance, the other distance can be found without having to 

 examine the focus on the ground-glass ; the only thing to do being to divide the 

 scale called " the unit of focal distances," in any number of parts corresponding in 

 an inverted ratio with the progression of distances in front of the glass. 



