NATURAL PHILOSOPHY. 127 



repeatedly reflected in the mirrors at equal intervals along the 

 circumference of a mathematically true circle, the centre of which 

 is the intersection of the mirrors. In all positions whatever, of 

 an object viewed by reflection with two plane mirrors, this actu- 

 ally remains true. The object appears the first of a series of 

 images arranged at equal intervals round the circumference of a 

 circle, the centre of which is the jooint of intersection of the mir- 

 rors with each other, or of the images, if need be, produced. 

 Of tlie two plane mirrors with which the cycloscope is entirely 

 composed, the front mirror is half silvered, and it is brought into 

 any convenient inclination with the entire mirror behind it, by 

 turning a screw. The parts of a I'evolution of the screw corre- 

 spond to minutes of a degree of inclination. When the last 

 chained peg of the straight line immediately preceding a railway 

 curve is seen directly through an eye-hole in the centre of the 

 entire mirror, its successive combined reflections at the same time 

 meet the eye at equal tangential angles, and trace out a circle, the 

 direction of the intended curve. The curve can then be set out 

 by pegs placed at equal chained distances apart, in the direction 

 of the combined reflections. By this means several points of a 

 railway curve can be set out at one sight, and the necessity of 

 repeated removal and readjustment of a theodolite in the ordinary 

 mode of setting out railway curves is avoided. The instrument, 

 which is made by Mr. Stanley, London, resembles a pocket-sex- 

 tant in being also a small and portable construction for measur- 

 ing distances and angles of moderate width. — Intellectual Ob- 

 server, May, 1866. 



OPTICAL DELUSION. 



Many of our readers will, no doubt, recollect "Eidos ^ides," 

 which was performed at Her Majesty's Theatre during the win- 

 ter. It has been made the subject of a patent by the inventor, 

 Mr. Maurice, from whose specification we learn the manner in 

 which this clever delusion is produced. It is jDerhaps necessary 

 to say that it consists in causing an actor, or an inanimate object 

 which is in full view of the audience at one moment, to disappear 

 instantly, and then to reappear with the same rapidity. The 

 means by which this is accomplished are very simple, and are, to 

 some extent, similar to those used in exhibiting " Pejjper's Ghost." 

 A sheet of plain unsilvered glass is placed upon the stage, either 

 upright or inclined at a suitable angle, at the place where the ac- 

 tor or object is to disappear. This glass is not perceived by the 

 audience, and it does not interfere with their view of the scenery, 

 etc., behind the plate. A duplicate scene, representing that part 

 of the back of the stage covered by the glass, is placed at the wing, 

 out of sight of the spectators. With the ordinary lighting of the 

 stage, the reflection of this counterfeit scene in the glass is too 

 faint to be observed ; but when a strong light is thrown upon the 

 scene, the stage lights being lowered at the same time, the image 

 becomes visible. This duplicate scene being an exact fac-simile 

 of the baclvground of the stage, the change is not noticed by the 

 audience, the only difference being that they now see by reflec- 



