336 SCIENTIFIC AMUSEMENTS. 



Such a mirror, if convex to the observer, will accordingly give a 

 reflection of the face of the natural length, but greatly diminished 

 width; on the other hand, if placed horizontally, the width of the 

 face will be natural, but the length greatly diminished. Analogous 

 opposite results will be obtained with hollow (concave) cylindrical 

 reflectors, provided the radius of curvature is sufficiently great to 

 enable the observer's face to be at a less distance from the mirror 

 surface than half that radius. 



The curved sides of a polished smooth-faced silver teapot, cup, 

 or goblet, &c., or a bright smooth block tin dish cover, &c., will 

 often show curiously distorted reflections formed in this way. On 

 the other hand, distorted drawings can be readily made which, 

 when seen by reflection from a mirror of appropriate curvature, 

 will no longer appear abnormal. Various amusing toys are con- 

 structed on these principles. 



Expt. 364. Spectral Illusions by Concave Mirrors. A large- 

 sized properly shaped well-polished concave mirror may be em- 

 ployed to produce a spectral illusion, somewhat on the same 

 principle as a magic lantern, but with still better effect. A highly 

 illuminated object placed at one of two conjugate foci will form 

 an image on a cloud of steam or other ground placed at the other 

 focus so as to receive it conveniently. This image, being inverted 

 with respect to the object, renders the production of stage phantoms 

 and illusions by means of a concave mirror somewhat difficult, 

 especially when the spectre is required to be in motion ; hence 

 the employment of such optical appliances is somewhat limited as 

 compared with other methods. But it appears to be very probable 

 that the properties of concave mirrors in this respect were known 

 to the ancient soothsayers and utilised in their oracles, &c. ; the 

 more so as, by properly arranging the relative position of object 

 and image, a magnified image can be obtained, so that the reflec- 

 tion of a man of ordinary stature would indicate a spectre of heroic 

 proportions. 



Expt. 365. Lenses of various kinds, and their Foci. A lens 

 means a portion of a transparent medium bounded by two inter- 

 secting surfaces of regular curvature, and so arranged that the light 

 enters on one surface and issues at the other ; one surface may be 

 plane (i.e., part of the surface of a sphere of indefinitely large 

 radius). According as the lens is thickest or thinnest in the 

 middle, it is spoken of as convex or concave respectively ; although 

 it may happen that one surface is actually convex and the other 

 concave this definition is adhered to, the lens being said to be 

 convex when the curvature of the convex surface is sharper (i.e., 

 the radius of curvature is less) than that of the concave surface ; 



