PHYSICS. 235 



Its upper end communicated with a closed vessel containing 

 ether or some other volatile liquid. Some water being placed 

 in the bottle, the ether was allowed to run on to the paper 

 strips. Here evaporating, pressure was generated in the in- 

 terior of the bottle, which threw a jet of the water to a con- 

 siderable height, thus doing work without the addition of 

 heat. 



LIGHT. 

 1. Reflection and Refraction. 



Masse has called attention to the explanation given many 

 years ago by Arago and Babinet of the phenomena exhibit- 

 ed by the so-called Japanese magic mirrors. These mirrors 

 are made of an alloy of copper and tin, are circular in form, 

 are from one eighth to one tenth inch in thickness, and have 

 Chinese or Japanese characters in strong relief on the back. 

 When sunlight is reflected from them on a wall, the charac- 

 ters appear. Since these mirrors are cast, they are not equal- 

 ly dense in all parts ; and hence in the operation of polishing 

 they become concave or convex over the characters in relief, 

 and these characters are therefore shown in the reflection. 

 Notwithstanding this entirely sufficient explanation proved 

 a year or more ago by President Morton, of the Stevens In- 

 stitute of Technology, by polishing the letters S. I. T. with a 

 little rouge on his finger on an ordinary Japanese mirror 

 showing no characters on reflection, and obtaining these let- 

 ters in the reflected image the magic mirror is brought for- 

 ward every few years as a phenomenon entirely inexplicable 

 by science. 



Hoffmann has devised a new form of camera lucida, which 

 seems to be an improvement upon the ordinary instrument. 

 In place of a total reflection prism he uses two mirrors, one 

 metallized, the other plain, placed at a fixed angle. The lat- 

 ter mirror transmits the rays coming from the pencil, and at 

 the same time reflects a part of the rays coming from the 

 object to be drawn, and which have already been reflected 

 from the metallized mirror. A neutral tint glass or a set of 

 lenses may be attached to the apparatus for si)ecial kinds of 

 work. 



Gariel has devised an ingenious abacus or chart by which 

 the relations of the conjugate foci and the principal focus of 



