EECENT PROGRESS IN PHYSICS, 429 



rific rays is absorbed witbout sensibly lessening tbe intensity of tbe 

 ligbt. 



The image of tbe arc of ligbt itself, as produced at/ by tbe concave 

 mirror, may be taken as tbe object to be magnified tbrougb tbe lens, but 

 then tbe magnifying power must be lower. Thus, a highly magnified 

 image of the arc of light is obtained I upon the white screen, and all 

 the phenomena accompanying it can be observed with ease. 



But the magnified image of tbe galvanic arc of ligbt can also be 

 produced by much simpler means ; it has only to be brought directly 

 near tbe focus of a lens of 1 to 3 centimetre focal distance. Of course, 

 care must be taken that the carbon light is so inclosed that no light 

 falls upon tbe screen, except that which passes through the lens. 



§ 69. Galvanic illumination of mines. — The remarks on galvanic 

 illumination in general are also applicable to the proposal to illumi- 

 nate mines by the light produced by means of galvanism ; there is 

 scarcely much practical success to be expected. Because the galvanic 

 carbon light can be produced in a' vacuum and even under water. 

 Boussingault believed that it could be used instead of Davy's safety 

 lamp in mines where inflammable gases make open lamps dangerous. 

 But in mines an intense light at any one place is never wanted, but 

 a feeble one at many difierent places. The same objection is to be 

 made to De la Rive's proposal to pass in a hermetically closed glass 

 balloon the positive current from an upright carbon cylinder to a me- 

 tallic one placed vertically above it. — (Dingler's Polyt. Journal, 

 XCVIII, p. 153, and 232 ; Moniteur industr., 1845, No. 961 & 965.) 



Grove proposed, instead' of the carbon light, the ignition of a pla- 

 tinum wire by galvanism in a hermetically closed glass vessel. — 

 (Dingler's Polyte'cbnic Journal, XCIX, p. 201 ; Phil. Magazine, Dec, 

 1845, p. 442.) He gives to the wire the form of a spiral, and thus 

 employs a greater length of it in a smaller space ; and this arrange- 

 ment has the further advantage that on account of the less rapid cool- 

 ing the coiled wire is more intensely heated with an equal force of 

 current than the same wire when extended in a straight line. 



Grove does not give any details as to the length and diameter of 

 tbe wire used, or of the precise construction of the spiral. With two 

 to three constant elements a uniform light was obtained during several 

 hours. Grove experimented (and read) by this light ; but this notice 

 gives only a very imperfect idea of the illuminating power of the 

 apparatus. In mines this light will probably be sufficient, and the 

 proposition therefore appears to be a more practical one than the ap- 

 plication of the carbon light, especially on account of the small 

 number of constant elements required for tbe purpose. But still , even 

 this apparatus will be more expensive and complicated than Davy's 

 safety lamp, and consequently its general introduction into use is 

 scarcely to be expected. 



King uses, instead of tbe platinum spiral, a strip of very thin pla- 

 tinum foil,, and makes it incandescent by the galvanic current. — 

 (London Journal of Arts, June, 1846, p. 348 ; Dingler's Polytechnic 

 Journal, CI, p. 12.) 



§ 70. Tlie galvanic sparJc. — Most of the observers who have experi- 

 mented with .the galvanic arc have noticed that the two poles must first 



