REFLEXION OF LIGHT. 439 



attain. One extremity was then fixed in a clamp attached to 

 a wooden frame ; and to the other extremity was attached, by 

 a similar clamp, a metal weight, from which weight a wire 

 extended and dipped into a vessel of mercury. The whole was 

 arranged with care, so as not to bend or disturb the plane 

 surface of the platinum. The foil thus suspended was brought 

 opposite a vertical cleft in a window-shutter facing the meri- 

 dian, which cleft could be made of any convenient size by 

 horizontally moveable boards. The platinum-foil was placed 

 opposite the cleft, so as to receive a sunbeam ; a sheet of 

 white paper was arranged directly in the path of the reflected 

 beam ; and the distance of the paper from the platinum was 

 indefinitely varied during the experiment. Having accurately 

 marked the boundaries of the reflected beam and its intensity, 

 as far as the eye could judge, the platinum was made part of 

 the circuit of a voltaic battery, the intensity of which was 

 varied so as to produce effects on the foil varying from a 

 heat scarcely visible in the dark to incandescence up to the 

 point of fusion, or rather to the point at which the foil broke, 

 from its diminished cohesion ; for with a weight suspended, 

 although not more than barely sufficient to keep the foil 

 stretched, it always broke off at a temperature short of its 

 point of fusion. In none of these variations, however, was 

 there the slightest apparent difference in the reflected light on 

 the paper. Or if, as occasionally happened, the shape a little 

 changed during the progress of the experiment, it was fully 

 explained by the elongation dependent upon the heat, or by 

 the consequent removal of slight curvatures. 



A similar experiment was made with diffused daylight, and 

 with similar effects, also with the light from an argand 

 lamp. In the latter case, when the reflected beam was so 

 dim as to be interfered with by the light afforded by the in- 

 candescent platinum, the image was proportionately affected. 

 Still it preserved its character, and, as far as could be judged, 

 its intensity ; and it was only by a very high degree of incan- 

 descence and a very feeble incident light that the reflected 

 image seemed to merge in the direct light from the incandes- 

 cent body. 



I now brought my eye into the position where the paper 



