NATURAL PHILOSOPHY. 157 



ture, having its particular perspective; and, consequently, in order to obtain 

 a single vision, the eyes have to converge differently, to bring consecutively 

 in the centre of both retinas the different similar points of the two pictures 

 according to their horizontal separation on the ground glass, the criterion of 

 their respective distances. This alteration of the convergence of the optic 

 axis, according to the distances of the various planes, gives the same sensa- 

 tion of relief we obtain when we look at the natural objects, or at their pho- 

 tographic representations. There is a charm in the effects of this instrument 

 peculiarly agreeable to the senses; and we may congratulate M. Claudet 

 upon having made a very remarkable and pleasing discovery. 



ALMEIDA'S STEREOSCOPE. 



An important modification of the Stereoscope has recently been commu- 

 nicated to the French Academy by M. d'Almeida. With the common instru- 

 ment, only one observer at a time can view the relief. M. d'Almeida renders 

 it visible to several at a time, and at a distance of several metres. For this 

 purpose he causes two stereoscopic images to be reflected simultaneously* 

 on a screen ; as they are not identical, but only similar, the outlines of the 

 one will intersect those of the other, and generate a confusion which can 

 only be obviated by making each eye see only one of the images. For 

 this purpose the inventor causes the luminous rays from each image to pass 

 through a glass of a different color, one red and the other green; whereby 

 one of the images will be reflected on the screen in red, the other in green. 

 Now, if the observer's eyes be provided with glasses of the above-mentioned 

 colors, the eye covered with a green glass will only see the green image, 

 while the other will only be visible to the eye protected by a red glass. The 

 moment this is effected, the relief appears; and if the observer shift his po- 

 sition laterally, the figure will appear to move in a contrary direction, which 

 adds to the illusion. M. d'Almeida proposes another plan, in which both 

 images are un colored, and each eye is made to perceive one image only by 

 rapidly intercepting the other from view by means of a revolving piece of 

 pasteboard, cut so as only to cover one of the images at a time by each half 

 revolution. As soon as the rotary motion acquires sufficient rapidity, the 

 figures appear in relief. 



ON THE CONSTRUCTION OF A HAND HELIOSTAT, FOR FLASHING 



SUN-SIGNALS. 



The following paper was presented to the British Association, 1858, by 

 Mr. F. Galton. A flash of sunlight from a looking-glass of a few inches in 

 the side, can be seen further than any terrestrial object whatever; and the 

 instrument about to be described shows how this remarkable power may be 

 utilized for the purpose of telegraphy. Heliostats are used in all government 

 surveys, and their power is well known in penetrating haze, and their utility 

 in requiring no "sky line." They were also habitually employed by the 

 Russians, for telegraphy, during the Crimean war. But all heliostats that 

 have been hitherto used have been fixtures of large dimensions; commonly, 

 a shaded screen, with an aperture in it, was placed at many yards from the 

 signaller, who stationed himself in such a way, that, when he could see the 

 play of his fla<h about the hole in the screen, he might be sure that some of 

 the rays which passed through the aperture would be visible at the distant 

 station. At other times a polished ring was used for the same purpose as 



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