ANNUAL OF SCIENTIFIC DISCOVERY. 



with a camera constructed with tv.'o lenses, or consecutively with a camera 

 with one moveable lens. The lenses of the stereoscope, besides magnifying 

 the pictures, are so placed as to unite certain similar points of them, thus 

 relieving the eyes of too great effort in uniting them entirely by convergency 

 of the axes. 



The means above alluded to, by which we are enabled to judge of differ- 

 ential distances, are of course much diminished by the distance that the 

 objects viewed are removed from us. Our consciousness of different dis- 

 tances by distinctness is diminished through decrease of light. Our judg- 

 ment, through change of focus, is diminished in consequence of the paral- 

 lelism of rays from distant objects being so nearly the same as to require 

 but little change in refracting them to a point on the retina. And lastly, the 

 binocular effect is in a great degree impaired through the identity of distant 

 views when seen from positions only separated by a base of 2 1-2 inches. 

 Xature has thus observed her usual economy in providing for our necessities 

 alone, it being of little comparative importance to us generally to be 

 acquainted with the relative positions of distant objects; whereas our per- 

 sonal convenience, and even safety, depend greatly upon our knowledge of 

 those near at hand. AVe are therefore provided with much more ample 

 means of determining the latter than the former. Jour. Franklin Institute. 



TELESTEREOSCOPE. 



Prof. Helmholtz has recently published a desci'iption of an instrument 

 which he calls a Tele-stereoscope (telescopic stereoscope), the object of which 

 is to present, stereoscopically united, two pictures of a landscape corre- 

 sponding to two points of view, whose distances considerably exceed the 

 distance between the two eyes. The stereoscopic power of the eyes is 

 small, because the distance between them is small; by the instrument it 

 is widened, and the effect which a stereoscope produces in a picture of a 

 landscape is thrown over the landscape itself. The instrument is made up 

 of four mirrors and two eye-glasses. Two mirrors placed, alike, at an angle 

 of 45, one to the right and the other to the left, receive the rays of light 

 from the landscape. These mirrors throw the rays horizontally towards 

 one another, to two oblique mirrors, Avhich throw the rays through the eye- 

 glasses to the eyes. In a window, place on either side, say three or four 

 feet, or the width of the window apart, a mirror, at the angle stated, to 

 receive the rays from outside, the planes of the two mirrors converging, of 

 course, to a point in the room. The mirrors will have the position of the 

 half-opened shutters of the window. The rays from, the scene outside, on 

 reaching them, will be thrown parallel to the window, those of one mirror 

 towards the other. Now, by placing at the middle of the window two 

 smaller mirrors, meeting like the legs of a V, but at an angle of 90 ', and 

 facing in the room, the rays will be thrown into the room ; and if these two 

 mirrors are not too large, or are properly placed, the rays will have just the 

 distance apart required to pass into the eyes. A box or frame may enclose 

 the mirrors, and a couple of lenses be inserted as eye-pieces, and the effect 

 thereby be improved, though the lenses should have a focal length of 

 thirty or forty inches. The mirrors should be made of the best plate glass. 

 The size may be much larger than the breadth of a window, although not 

 to any very great advantage. 



To see near objects in the telestereoscope, the reflectors must be turned 



