400 Prof. Draper on the Production of Light by Heat. 



It is not to be forgotten, however, that in the case of light we 

 necessarily measure its effects by an apparatus which possesses 

 special peculiarities. The eye is insensible to rays which are not 

 comprehended within certain limits of refrangibility. In these 

 experiments, it is requisite to raise the temperature of the plati- 

 num almost to 1000° before we can discover the first traces of 

 light. Measures obtained under such circumstances are depen- 

 dent on the physiological action of the visual organ itself, and 

 hence their analogy with those obtained by the thermometer be- 

 comes more striking, because we should scarcely have anticipated 

 that it could be so complete. 



Description of the apparatus employed in the foregoing expert- 



ments. 



foil 



The source of light is in all instances a slip of platinum 

 1-35 inch long, and ^ t th of an inch broad, ignited by the passage 

 of a voltaic current, and placed in such a position that its dilata- 

 tion could be measured by the movements of an index over a 

 graduated scale. 



In fig. 7, a b represents the slip of platinum, the upper end of 

 which is soldered to a stout and short copper pin a, firmly sunk 

 in a block of wood c, which is immovably fastened on the basis 

 d d of the instrument. A cavity e, half an inch in diameter, is 

 sunk in the block c, and into this cavity the pin a projects ; so 

 that when the cavity is filled with mercury, a voltaic current may 

 be passed through the pin and down the platinum. 



Fig. 7. 



The other extremity of the platinum b is fastened to a del- 



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