It is evident that the perpendicular distance of P Q from L or R, (which we will call F,) will depend upon the size of the angle ALB, or ARB • for the smaller that angle is the further the plane PQ may be placed from L and R, and therefore the more easy it will be come for the images a, b,c, to be seen distinctly by persons of ordinary vision. Most persons can see a thing distinctly at a distance of 8 inches. If then F=8, the angle ALB may be about 16°.
Now we come to the principles of the stereoscope.
Suppose we place at each of the stations L,11, a photographic camera, the lens of which is 8 inches focus, and adjusting these ca meras with their awes strictly parallel, take the pictures of A,B,C, (including an angle of 16°). Then when these pictures are properly mounted they will be identical with the perspective views of A,B, C, upon the plane P Q ; and if placed in a box having a partition in the middle and holes to look through at P and Q at a dis tance of 8 inches from the pictures, the spectator on looking through the holes will perceive a single image of the points A,B, C, at their true distance and in their true relative positions.
A box so constructed may be called a " Simple Stereoscope," because it does not involve the use either of lenses or reflectors, and the pictures properly taken and viewed in it have the inexpressible charm of truthfulness.
Of what use then, it may be asked, are lenses and reflectors ? To this question we will now endeavour to reply.
In the first place the simple stereoscope, when adapted to persons of ordinary sight, does not include an angular field of more than 16° ; which is in general too small. A photographic picture should include at least an angular field of from 25° to 30°. This can only be effec ted by bringing the pictures as near to the eyes as 5 inches, at which distance most persons find a difficulty in seeing them distinctly ; but this difficulty is overcome by placing a whole lens of 5 inches focus in each of the holes L,R, and viewing the pictures, (which must be taken with a lens of 5 inches focus,) through them. This instrument is called the " Lenticular Stereoscope." In the following description of it we shall recapitulate what has been said, and discuss the matter more fully.
The Lenticular Stereoscope.
This is an instrument for exhibiting a pair of stereoscopic pictures, which include an angular field of about 26°, and have been taken in a stereoscopic camera provided with lenses of 5 inches equivalent focal length, having their axes parallel, and mounted at a distance L R (=2-1. inches) from centre to centre. [The reader will observe that the stereoscopic camera described at page 67 of this work is calcu lated to exaggerate the relief of objects, because the stations are taken at 5 inches apart ; in all other respects however the principle of it is correct.]
The best construction of the Lenticular Stereosc,ope for exhibiting paper pictures, (kata-positives,) is shewn in the following figure, which scarcely needs explanation.
The whole lenses fixed in the frozt of the box are placed 2+ inches from centre to centre, and should not be less than 1 inch in diameter. They should be of 5 inches focus, and achromatized meniscus lenses with the hollow side outwards, in fa.ct the same lenses as those used for taking the pictures, which may be unscrewed from the camera, and screwed to the stereoscope. The back of the solid box should have holes in it as represented by the dotted lines, and also a parti tion in the middle. The holes should be 2i inches from centre to centre, and their diameter determined by trial. If the front box be 2 f,- inches deep, the back diaphragms should be a trifle more than 1 inch in diameter. A. set of diaphragms of different sizes to fit in the back of the front box would be a useful addition to the instrument. The inside of the front box should be properly blackened. The pictures may be circular, mounted 21 inches from centre to centre and about 2+ niches in diameter. By the centre of each picture is meant the point where the axis of the lens cuts it. In mounting the pictures this is the point to be considered, and no reference what ever is to be made to any of the objects in the pictures. These are always nearer together than 2i inches, when mounted. The moun ted pictures are fastened by springs to a slider which moves in a slit in the bottom of the stereoscope, so that the distance of the pictures from the lenses may be varied to suit different sights, or for trying experiments ; the proper distance, however, being 5 inches, which is the focal length of the lenses. The pictures should be mounted upon black cardboard. In order to make the objects appear more distant than the cardboard, that is, in order to make them appear as if viewed through a circular hole nearer to the spectator, the following rule in mounting them should be attende,d to. Just before trimming the edges, mount them temporarily 2i inches from centre to centre, upon a piec,e of cardboard, with pins at the comers ; then measure the distance between the nearest object in each picture, (this will be less than 21 inches, probably not much more than 2-k inches,) and take a dimension a little leu than this ?or the diamet,er of the pictures. With half this diameter as radius, and the c,entre of the picture as centre of the circle, strike a circle on each picture, which will be the margin required.