NATURAL PHILOSOPHY. 173 



We can entirely confirm the truth of this statement, and a striking 

 illustration is to be found in two lochs in the northwest of Sutherland- 

 shire, separated only by a low ridge of land. In the one which is 

 full of dark moss water the trout are nearly black; in the other, 

 Avhere the bed of the lorli is limestone, and the water so clear that 

 you can see the bottom where it is 40 or 50 feet deep they are al- 

 most as silvery in color as sea trout. Loch Brora, too another 

 loch in the same country affords a further corroboration of the truth 

 of JMr. St. John's observations. That loch is divided into three 

 sheets oi' water, united by narrows, where the lake assumes the ap- 

 ]>e:ir:ince of a river. In the upper part, where the bottom is sand 

 and fine gravel, the trout are clear in color, with bright vermilion 

 spots : in the central division, where the bottom is not so clean, and the 

 water darker, they are also dark in color, and their spots are not so 

 bright ; while in the lowest division of the lake, where the bottom is 

 very muddy, the trout are quite black and ugly, though of a larger 

 size. Erasers Magazine. 



MOTIONS OF THE EYE IN RELATION TO BINOCULAR VISION. 



The motions of the eye can be compared to those of our arm. We 

 can raise and lower the visual line, we can turn it to the left and to the 

 right, we can bring it into every possible direction throughout a certain 

 range as far, at least as the connections of the eyeball permit. So far 

 the motions of the- eye are as free as those of the arm. But when we 

 have chosen any determinate direction of the eye, can we turn the eye 

 round the visual line as an axi^ as we can turn the arm round its longitu- 

 dinal axis? This is a question the answer to which is connected with a 

 curious peculiarity of our voluntary motions. Yes: there exist muscles 

 by the action of which those rotations round the visual line can be per- 

 formed. But when we ask, can we do it by any act of our will ? We 

 must answer "No." We can voluntarily turn the visual line into every 

 possible direction, but we cannot voluntarily use the muscles of our eye 

 in such a way as to turn it round the visual line. Whenever the di- 

 rection of the visual line is fixed, the position of our eye, as far as it 

 depends upon our will, is completely fixed and cannot be altered. 



A NEW STEREOSCOPE WITHOUT LENSES. 



In the Nuovo Lincei, M. Volpicelli describes a new stereoscope in- 

 vent d by him in April, 1854. This stereoscope is without mirrors 

 or lenses, and consists of a rectangular horizontal box, whose propor- 

 tions are as follows: hight 11 centim. (4'5 inches); depth 20 centim. 

 (8 inches) ; length 62 centim. (24-8 inches). The two stereoscopic pictures 

 are placed against the back of the box; in the front of the box two holes 

 are hored opposite the middle of the pictures. Two diaphragms made 

 of phtes of blackened wood or card-board, of the hight of the pictures, 

 are made to rotate around vertical axes corresponding to the front 

 edges; and are so adjusted that they allow the right eye to see only the 

 left picture, and the left eye the right picture. It follows that the rays 

 by which the pictures are seen cross each other within a certain space in 

 the middle of the box, and in this space, after a little effort, the eyes see 

 the object in relief. 

 15* 



