36 Optical Ghosts. 



eyes. Old writers were well aware of tlie fact tliat a plane 

 mirror could be so arranged that a person looking at it should 

 not see himself, but see something else, which might be behind 

 a screen, and quite out of his natural view. It is, indeed, very 

 easy to make a looking-glass show you objects quite out of 

 your line of vision, and one of the facets of a moderate-sized 

 diamond will easily enable you to see by reflection any object 

 in a room, when you appear only to be looking at the finger 

 that carries the ring in which it is set. 



Having made a few experiments with the looking-glass, 

 take a pane of window glass, or, what is better if you have it, 

 a piece of plate glass, the surface of which is more true, and 

 hold it upright on the table near a window. A few inches in 

 front of it place any small object on the table ; a lady's cotton 

 reel will do extremely well. Stand upright with your back to 

 the window, but leave room for the light to fall freely on the 

 top of the reel. Look slantingly down at the glass, and you 

 will see the image of the reel reflected by its surface, and 

 apparently as far behind as it really is before. The top on 

 which the light falls will be brilliant, and the part that is in 

 the shade will be reflected in shadow. Vary the experiment 

 by placing a second reel, exactly like the first, as much behind 

 the glass as the other is placed in front of it. You then have two 

 reels presented to your eye, one actual, and the other spectral, 

 and you can, as Mr. Dircks remarks of a similar case, so 

 arrange the objects, and your position, that the image reflected 

 from the surface of the glass shall exactly correspond vrith the 

 outlines of the real reel seen through the glass. If you put 

 any small article on the top of the reel in front of the glass, or 

 -nine one else puts a similar object on the top of the reel 

 behind the glass, the optical effects will be the same. 



Now make a third experiment. Put a box, or thick book, 

 in front of you, so that you cannot see the reel, when placed 

 on the table just under its edge. Then hold the glass a little 

 way off, and upright as before, so that you see it from top to 

 bottom. You may then obtain a reflected image of the reel, 

 which the book conceals, and if a strong light were thrown 

 upon it, the image would be as sharp, distinct, and apparently 

 solid as tlie reality. 



Thus this kind of optica] uhost is very easily made, and 

 .Mi'. Dircks suggests a few effective tricks. We have not dwelt 



at any length upon verbal explanations, because everybody 

 can make the simple experiments suggested, and they will 

 explain the matte]- mueh better than a lengthened essay could 



effect. We ought, bowerer, to add, that Messrs. Home and 

 Thornthwaite supply a portable apparatus, by which the 

 Dircksian ghosts can be easily and strikingly shown. 



