18 Sir David Brewster on Nero Stereoscopes. 



them before they again view the drawings. It exists chiefly with 

 short-sighted persons, for whom the stereoscope may be con- 

 structed with concave semilenses or quarters of lenses, placed as 

 in fig. 16 j and when there are only two drawings, it may be 

 prevented by a partition, which hides the right-hand drawing 

 from the left eye, and the left-hand drawing from the right eye. 

 The instrument, as fitted up for use, is shown in fig. 2, where 

 ABCD is a frame of tin or wood, consisting of an upper and a 

 lower plate, and two ends, AB and CD. The semilenses are 

 placed in CD, with an opening for the nose at NN, a part of 

 the lower plate being cut away for this purpose. The three dis- 

 similar drawings, as shown at C, fig. 4, are placed in the end 

 AB, and are illuminated by the light which enters by the two 

 open sides, AC, BD*. If the drawings are upon thin or trans- 

 parent paper, or are executed as transparencies like the diagrams 

 used in the magic lantern, the box ABCD may be closed, and 

 the light admitted only through the end AB. In the form shown 

 in fig. 2, where the drawings slide into an open frame, either 

 opake or transparent figures may be used. It is often convenient 

 to have the drawings separate, so that, like the semilenses, they 

 may be made to approach to or recede from one another ; and 

 when the drawings are thus separate, we can obtain the arrange- 

 ment at B, fig. 4, from the drawings at A, or all of them from 

 the three drawings at C. 



While the semilenses thus double the drawings and enable us 

 to unite two of the images, they at the same time magnify them, 

 — an advantage of a very peculiar kind, when we wish to give a 

 great apparent magnitude to drawings on a small scale, taken 

 photographically with the camera. But while the magnifying 

 power of any lens is the same through whatever portion of it we 

 look, its prismatic angle varies with the distance of that portion 

 from the margin. In the semilens LL, for example, fig. 3, the 

 prismatic angle is a maximum at the margin A, less at A', and 

 still less at A", so that when the drawing is very small, we can 

 double it, and refract it sufficiently by looking through A", when 

 larger through A', and when larger still through A. By using 

 a thicker lens, without changing the curvature of its surface, or 

 its focal length, we can increase the prismatic angle at its margin, 

 so as to produce any degree of refraction that may be required 

 for the purposes of experiment, or for the duplication of large 

 drawings. 



* It is sometimes more convenient to close the sides, and leave the upper 

 and under side's open, or we may cut off a circular segment from its upper 

 and lower plate, as shown in fig. 2. The use of this opening in the lower 

 plate is to illuminate the drawings when we turn the stereoscope and figures 

 upside down, which increases the relief in a surprising degree. 



