326 Dr. Herschel's Experiments, &c. 



No. 3, The standard thermometer. 



D, The little moveable pasteboard screen. 



Plate XVI. Fig. 2. 



AB, The front, plated with iron, that it may bear to be 

 exposed close to the bars of a chimney fire. 



C, The concave mirror. 



D, The plain mirror. 



E, The lens. 



No. 1, The thermometer in the focus of the lens. 

 No. 4, The standard thermometer. 



F, A circular opening in the front plate AB, for admitting 

 the rays of heat to fall on the concave mirror C. 



m, The first focus of the rays, from which they go on 

 diverging, to the small mirror, and to the lens ; which brings 

 them to a second focus, on the ball of the thermometer No. 1. 



