Sir David Brewster on New Stereoscopes. 25 



6. Method of uniting Similar or Dissimilar Figures. 



Upon a piece of glass MN, fig. 14, place a veiy small circle 

 of white paper D, and let A, B, C be similar patterns which we 

 wish to unite, A with C, or A with B. Hold the piece of glass 

 MN in both hands, and at such a distance from the eyes that, 

 when with the left eye L, and shutting the right eye, we see 

 the circle 1) covering C, we also, upon opening the right eye B, 

 see w r ith it the circle D covering A. By continuing for a short 

 time to look at the circle D with both eyes open, we shall see 

 the patterns all united, and the wall or plane which contains 

 them situated at the same distance from the eye as the circle D. 

 If there are one or more intermediate patterns, such as B, the 

 piece of glass MN must be held further from the eyes in order 

 to unite A with B instead of A with C. Those who acquire in 

 this way the art of uniting dissimilar and similar figures, will 

 not require in any case the aid of the stereoscope, unless when 

 there is only one figure or object ; in which case they must have 

 recourse to the total-reflexion stereoscope, in order to convert the 

 single figure into a solid, by creating and uniting with it its op- 

 posite or reflected image. 



7. Method of Drawing on a Plane the Dissimilar Representations 

 of Solids for the Stereoscope. 



Let L, R, fig. 15, be the left and right eye, and A the middle 

 point between them. Let MN be the plane on which an object 

 or solid, whose height is CB, is to be drawn. Through B draw 

 LB, meeting MN in c; then if the object is a solid, with its 

 apex at B, Cc will be the distance of its apex from the centre C 

 of its base, as seen by the left eye. As seen by the right eye R, 

 Cc' will have the same value, but c' will lie on the left side of C. 

 Calling E the distance between the two eyes, and h the height 



BC of the solid, we shall have AB :h= -r- -. Cc and Cc = ^ . _ , 



2 2AB 



which will give us the results in the following table, AC being 



= 8 and E = 2i inches : — 



