174 ANNUAL OF SCIENTIFIC DISCOVERY. 



If after the relief is seen the diaphragms are turned around their axes, 

 so as to rest against the sides of the box, and no longer intercept the 

 view the eyes still continu'ng to view the picture in rell-f a very inter- 

 esting physiological phenomenon will be seen. There Avill be seen three 

 pictures one in the middle of the instrument, in relief, the others along- 

 side, and by no effort of will can the middle picture be made to disap- 

 pear. If the pictures be of complementary colors, they will be seen of 

 their own colors, and the relief will be white. Those who are not accus- 

 tomed to observation of optical phenomena will require some attention 

 to see the relief at first, but it will generally be found by looking atten- 

 tively at about the middle* of the box. When once seen it mav be 



* 



recovered without anv difficulty. Cosmos. 



* 



DESCRIPTION OF A NEW OPTICAL INSTRUMENT CALLED THE 

 "STEREOTROPE," BY W. T. SHAW, OF ENGLAND. 



This instrument is an application of the principle of the stereoscope 

 to that class of instruments variously termed thaumatropes, phanta- 

 scopes, phenakistoscopes, &c., which depend for their results on " per- 

 sistence of vision." In these instruments, as is well known, an object 

 represented on a revolving disc, in the successive positions it assumes 

 in performing a given revolution, is seen to execute the movement so 

 delineated ; in the stereotrope the effect of solidity is superadded, so 

 that the object is perceived as if in motion and with an appearance of 

 rc'lief as in nature. The following is the manner in which I adapt to 

 this purpose the refracting form of the stereoscope. 



Having procured eight stereoscopic pictures of an object of a 

 steam engine for example in the successive positions it assumes in 

 completing a revolution, I affix them, in the order in which they were 

 taken to an octagonal drum, which revolves on a horizontal axis be- 

 neath an ordinary lenticular stereoscope, and brings them one after 

 another into view. Immediately beneath the lenses, and with its axis 

 situated half an inch from the plane of sight, is fixed a solid cylinder, 

 four inches in diameter, capable of being moved freely on its axis. This 

 cylinder, which is called the eye-cylinder, is pierced throughout its 

 entire length (if we except a diaphragm in the centre inserted for ob- 

 vious reasons) by two apertures, of such a shape, and so situated rela- 

 tively to each other, that a transverse section of the cylinder shows 

 them as cones, with their apices pointing in opposite directions, and 

 with their axes parallel to, and distant half an inch from, the diame- 

 ter of the cylinder. Attached to the axis of the eye-cylinder is a 

 pulley, exactly 5 the size of a similar pulley affixed to the axis 

 of the picture-drum, with which it is connected by means of an end- 

 less band. The eye-cylinder thus making four revolutions to one of 

 the picture-drum, it is evident that the axes of its apertures will re- 

 spectively coincide with the plane of sight four times in one complete 

 revolution of the instrument, and that, consequently, vision will be 

 permitted eight times or once for each picture. 



The cylinder is so placed that at the time of vision the large ends 

 of the apertures are next the eyes, the effect of which is that when 

 the small ends pass the eyes the axes of the apertures, by rea- 

 son of their eccentricity, do not coincide with the plane of sight, and 

 vision is therefore impossible. If, however, the position of the cylin- 



