332 M. MELLONI ON THE POLARIZATION OF HEAT. 



nary heat ; the rest pur-^ues its waj' without losing its radiating state, is 

 afterwards dispersed, and falls upon a second lens with a shorter focus, 

 placed beyond the first at a distance precisely equal to its own principal 

 focal distance. The rays received by this second lens in a state of di- 

 vergence are parallel to each other when they leave it ; and form a pencil 

 of condensed heat, which enters the thermoscopic case, and finally 

 reaches the pile, which stands at a suitable distance from the aperture. 

 The section of the pencil being a little less than that of the pile, all its 

 parts concur in the production of the thermoscopic effect, and thus we 

 lose the calorific effect of none of the rays that issue from the polar- 

 izing system. 



It is very important to observe, that the common centre of the two 

 superposed plates of tourmaline is not placed exactly in the common 

 focus of the two conjugate lenses, but a little nearer to the second, in 

 order that the portion of heat absorbed by tliose plates, and radiated on 

 the second lens, may be necessarily refracted in diverging rays whose 

 action becomes weaker, and is completely destroyed at a short distance, 

 without influencing the thermoscopic body, which is thus affected only 

 by the heat arising from direct transmission. That this condition is 

 actually fulfilled may be ascertained by blackening the tourmalines, or 

 by substituting for them any other plates well covered with lampblack ; 

 for the index of the galvanometer reassumes its natural position in equi- 

 librio, and retains it whether the communication with the calorific 

 source be established or intercepted. 



By this simple contrivance we cause very minute plates of tourmaline 

 to transmit a bundle of rays almost as broad as the surface of the first 

 lens, and then bring all the emergent rays, and these alone, pure, and 

 unmixed with even the smallest particle of the caloric derived from the 

 heating of the plates, to produce their effect on the thermoscope. 



Combining a lens of two inches and a half in diameter and three 

 inches in the focus, with a lens of fourteen lines, I obtain a quantity of 

 rays emerging from the tourmalines, such as, in several instances, pro- 

 duces a deviation of the needles, amounting to between 60° and 80°, at 

 the distance of a metre from the small flame of a Locatelli lamp fur- 

 nished with a reflector. This energetic action, though necessary for 

 the experiments which I had in view, is then too great ; but there 

 is an easy way of reducing it as much as we wish : we have only to 

 render the rays more or less divergent by a proper approximation of the 

 lenses. 



The plates of tourmaline are adjusted to the central part of two pa- 

 rallel covers of cork filling the interior of a round box, which is suffi- 

 ciently shallow, has a small circular aperture at the centre, and is sup- 

 ported at the proper height by a metallic screen with a similar aperture. 



