C. GENERAL PHYSICS. 191 



numerous observations tending to disprove the theories of 

 Scherffer, and of those who have followed him, as also the 

 theories of Thomas Young and his followers. Bull. Ac. Hoy. , 

 Belgiqiie, 1875, 100. 



REFLECTION OF THIN FILMS. 



Govi lias made a happy application of that principle in 

 optics by which thin films can at the same time reflect and 

 transmit rays of light according to their angles of incidence. 

 He applies a film of gold to the hypothenuse of a right-an- 

 gled prism of glass; the film allows direct rays to pass 

 through the prism, while the latter reflects the oblique rays 

 coming through the microscope. By placing this prism 

 obliquely upon the ocular of a microscope, the magnified 

 image is reflected upon a sheet of paper, where it can be 

 drawn by the observer who looks through the gold film. 

 JRev. Scl, 1874, 167. 



SIEMENS's ELECTRICAL PYROMETER, AND DIFFERENTIAL VOL- 

 TAMETER. 



We had occasion some years ago to give some account of 

 the very elaborate investigations made by Weinhold in refer- 

 ence to the reliability of the various methods employed for 

 measuring very high temperatures accurately, and to call 

 attention to the fact that his researches fully substantiated 

 the claim of Dr. Siemens that the electrical pyrometer, as 

 constructed by him, was a thoroughly reliable instrument. 

 It is now a pleasure to be able to refer to the very impor- 

 tant memoir of Dr. Siemens himself, just published in the 

 Journal of the Society of Telegraph Engineers. This me- 

 moir, which was in part delivered as a lecture in 1871, has 

 been delayed in its publication, owing to the innumerable 

 interruptions experienced by the author in consequence of 

 his professional duties. The fullness of its details shows how 

 large a series of experiments Dr. Siemens undertook to sat- 

 isfy himself of the accuracy of his method of measuring tem- 

 peratures. 



His memoir consists, first of all, of a very suggestive chap- 

 ter on the influence of temperature upon the electrical resist^ 

 ance of metallic conductors, which he expresses as a function 

 not only of the temperature reckoned from the absolute zero, 



