CONCLUDING REMARKS.
It must not be thought that the stereoscope is merely an entertaining toy. Science has succeeded in turning it to account for varied purposes. In as tronomy, an adaptation of the instrument is used to determine the distance and con in Fig. 858. On turning the handle at the side, a rod working on a crank pushes the camera backward and forward upon parallel bars, while a revolving piston rod operates the spindle of the camera and follows its motion along the bars. Film pictures obtained by this apparatus depict the illusion of motion with the additional element of solidity, the result being due to the fact that the retentive power of the mind is superior as regards duration of impression to that of the eye. Thus, after the eye has lost the impression received, it is retained in the mind, and persists there till supplemented with the succeeding phase from a similar point of view. In other words, the internal oscillation of images upon the retina, due to the circumstances of binocular per ception, instead of being produced in the eye by the varying direction of the axes, is produced externally upon the lantern figuration of celestial bodies by means of photographs taken from widely separated view points and combined stereoscopically, a calculation of distances being possible by an ingenious formula depending on the parallax or angle of view as compared with measurements on the two pictures.
The educational value of the stereoscope is obvious, where a clear and definite idea of objects and places not readily accessible is desired. In medicine, also, the stereo scope has sundry uses ; as, for example, in the curing of squint or "cross-eye." It is surprising that this most instructive and recreative instrument is not more popular, considering its inexpensive character. It has, however, lately received more atten tion, and seems to be growing in favour.
very sensitive to vibration, so that the ex posure should not be prolonged. Dead-black backgrounds are, as a rule, unsuitable.