Nipher — Physics During the Last Century. 121 



farmers are everywhere building their own systems and owning 

 their instruments. The value of such a service to a farming 

 community is very great, and the financial advantage is not 

 its most valuable feature. 



The discovery of Roentgen in 1895 has taxed the ingenuity 

 of every man who has sought to explain the nature of the 

 X-ray. It was a discovery which lays hold of the secrets of 

 the ether and the atom, and is likely to lead to results which 

 as yet cannot even be conjectured. Becquerel and Madam 

 Curie have found invisible radiations from various substances, 

 which possess all the essential properties of the X-rays. It 

 is said that Madam Curie is so saturated with radio-active 

 matter, that she is barred from all laboratories where electrical 

 work is being done. In her presence all electrified bodies are 

 discharged. Another great discovery of the last decade was 

 made by Zeeman of Holland. He found that if an incandes- 

 cent gas whose spectrum is being examined, be placed in a 

 strong magnetic field, the bright lines of the spectrum are 

 resolved into component lines, which are plane polarized. 

 Of the D lines given by sodium, D x becomes four lines, and 

 D„ becomes six. Lorenz has shown that this phenomenon is 

 fully accounted for by the electromagnetic theory of light. 



Even so fragmentary a review as this should contain some 

 reference to the science and art of photography, which is 

 wholly a product of the last century. Previous to the daguerreo- 

 type process, which was due to Daguerre and Nice'phore de 

 Niepse, there was a process due to the latter, which yielded a 

 permanent image on a metallic plate covered with an asphalt- 

 urn varnish, which was then developed by means of a solvent. 

 The time of exposure was from three to eight hours. The 

 picture was in faint relief, the parts which had been most acted 

 upon by the light being least acted upon by the developer. 



The daguerreotype which was produced in 1839, was a pic- 

 ture, on a silver plate, or a copper plate coated with silver. 

 The sensitive layer was formed by holding the plate in iodine 

 vapor, and the image was then developed by holding the plate 

 in mercury vapor. The vapor of mercury condensed upon 

 the plate in proportion to the light action, so that the picture 

 is a mercury amalgam. The plate was fixed by means of 



