428 



EECENT PROGRESS IN PHYSICS. 



Fig. 62. 



In an economical point of view, therefore, the galvanic illumina- 

 tion of streets, halls, theatres^, &c., does not appear advantageous. 

 But there is yet another difficulty ; the management of the battery 

 and of the whole apparatus is too complicated to be confided to such. 

 persons as generally have charge of the illumination ; tlie carbon 

 points are continually changing, and their position, therefore, must 

 be continually regulated in order to keep the light uniform and pre- 

 vent its extinction. It is difficult to accomplish this regulation by 

 mechanical means, though different contrivances have been proposed 

 for the purpose, Le Molt, for instance, obtained a patent in England, 

 in 1848, for an apparatus for galvanic illumination, in which carbon 

 disks, with the form represented in fig. 62, take the place 

 of the points. Two of these disks are placed with their 

 sharp edges opposite each other ; their axes rotate uni- 

 formly by means of clock work, and their distances are 

 regulated by a metallic spring. 



It is therefore scarcely to be expected that the applica- 

 tion of galvanism to public illumination will have any 

 practical success. But Donne and Foucault have obtained very 

 favorable results from their experiments, in which the galvanic carbon 

 light was substituted for the incandescent lime in the so called gas 

 microscope. 



A tolerably complete description of the photo-electric microscope of 

 Donne and Foucault maybe found in the 4th edition of Pouillet, Ele- 

 ments de Physique Experimentale, &c., vol. II, pp. *746. We can 

 here only indicate the most essential parts of the apparatus. The 

 luminous arc is produced between sticks of carbon cut from the hard 



carbon of gas retorts ; they are made 



^''^s-G3. jj2 the shape represented in fig. 63, the 



negative electrode being pointed and 



n l» the positive blunt. These carbon pieces 



^^^^^H^^^SB- ^^^^^M apg gQ }^gl(j tl^r^^ their position can 



easily be regulated. 

 A general idea of the arrangement of the illuminating apparatus 



of this microscope may be gathered from the diagram fig. 64. a is a 



concave mirror of an aperture of about 

 i decimetre, and a radius of 1.6 de- 

 cimetre. The carbon light is at b, a 

 little nearer to the mirror than c, the 

 centre of its curvature, and somewhat 

 higher, so that the rays emanating 

 from b are collecl'ed at /, v/here the 

 ininute object to be magnified is in- 

 tensely illuminated. The system of 



lens through which the magnified 



image of the object is thrown upon a 



screen 4 to 5 metres distant is precisely the same as in the solar 



microscope. 



To diminish the great heat at /, a vessel is placed between the 



mirror and b, the sides of which are made of polished plate glass; it 



it is filled with a solution of alum by which a great part of the calo- 



Fig. 64. 



