142 ANNUAL KECOliD OF SCIENCE AND INDUSTRY. 



that if a thin film of silver be deposited on a glass surface, 

 this metallic film will, if of extreme thinness, give to the 

 reflected lio-ht somewhat of the character of lio^ht reflected 

 from a metallic surface ; while, if the thickness of the film be 

 comparatively great, the 2:)i'operties of the reflected light be- 

 come simply those due to metallic reflection. The special 

 interest of the measures which M. Mascart has published lies 

 in the fact that by three different methods of measuring these 

 films he has arrived at results confirmino- -each other, and 

 showing that films of silver, wliose thickness is but one two- 

 hundredth part of the length of a wave of light, exert an ap- 

 preciable influence on the phase of the light that is reflected 

 from it, and that a film one quarter of a wave lengtli in 

 thickness gives ahiiost pure metallic reflection. The influ- 

 ence of the slightest deposit of oil, or any change in the 

 condition of the reflecting surface, is, of course, immediately 

 perceptible in these experiments. 6^, 1873, 868. 



LISSAJOUS' PHOXOPTOMETEK. 



Lissajous describes an extremely convenient method of 

 studying the small movements, either periodic or continuous, 

 of any body, at greater or less distances. His microscope, 

 having a vibrating object-glass, was described in 1857; but 

 he has now generalized the principle of the instrument, and 

 by putting the ocular, or the front lens of the ocular, into vi- 

 bration, he has much extended the usefulness of his invention. 

 The vibration of the lens is brought about by attaching it to 

 a vibrating sounding body. He has applied the phonopto- 

 meter to the study of bells and the qualities of their sounds, 

 to the vibrations in the interior of a stove, to the movement 

 of reed pipes, and to the velocity of projectiles. 6^,1873, 

 878. 



THE MOST TRUSTWORTHY PYROMETER. 



The great advantages Avhich those enjoy who pursue their 

 studies in the schools and nniversities of Germany are spe- 

 cially perceived when we examine the theses of the gradua- 

 ting classes and of their instructors, which are frequently pub- 

 lished in connection with annual programmes of lectures. 

 The last programme of the Technological School at Chemnitz 

 contains a most exhaustive investigation, both experimental 



