196 PROFESSOR FORBES'S RESEARCHES ON HEAT. 



through a diaphragm T, placed on one or other side of the prism (it does not 

 much matter which, as the beam which arrives at the pile is always much wider 

 than the second diaphragm t, placed there to admit only the central rays arriving 

 parallel to the line ac). The use of this diaphragm is, that a narrow enough 

 pencil of rays may be employed, to be independent of the variable breadth under 

 which the surface of the prism is presented to the incident beam. The usual di- 

 mension of this diaphragm was one inch in breadth, and one and a quarter in 

 height, but in some instances its breadth was reduced to three-eighths of an inch. 



56. The pile j) has its funnel-shaped orifice closed by a screen with a vertical 

 slit, an inch wide, in the direction of its axis. But there is a peculiarity in the 

 arrangement of the pile very essential to the success of these experiments, where 

 the pile itself is moveable, which I must not omit to mention. Its exposure to 

 currents of air would render the observations, when the pUe cannot be entirely 

 enclosed by a box or screen, very capricious in its action. I therefore adapted to 

 the end, bearing the conical reflector (II. 6), an adjustable wooden tube ?', con- 

 taining a rock-salt lens, which stiU farther increased its sensibility, and totally 

 protected it from aerial currents. 



57. The more important adjustments of the apparatus previous to use, are 

 these : 1. To place the surface a'V of the prism (Fig. 2) so as to form equal angles 

 with the sides of the lozenge Ky, K^, the point K being precisely above the angle 

 of the lozenge frame. To accomplish this, the prism rests upon a brass plate, 

 haAdng an adjusting motion concentric with that of the piUar (Fig. 1), on which 

 it rests. The adjustment was made by placing a piece of truly parallel mirror- 

 glass in the position of the posterior surface of the prism, suspending two plumb 

 lines in the prolongation of the lines ae, ad, and observing by the eye placed at 

 c whether the reflection of the other was seen in the direction ac, and adjusting 

 the brass plate before mentioned, bearing the mirror, until such was the case, 

 then making it fast by a clamping screw. 2. The next adjustment was to bring 

 the centre of the lens L into the line ac, which was done by placing a small flame 

 of a lamp in the position of the axis of the pUe p, and regulating the position of 

 the lens until the image of it feU exactly upon the prolongation of the Une ac, 

 the prism being so placed that the angles of incidence were almost perpendicular ; 

 (reflection at a mirror would have been preferable). 3. The adjustment of the 

 source of heat behind the lens is the next point. When the source is luminous, 

 it is done by causing the axis of the refracted cylinder of light to coincide with 

 the line ac ; when not luminous, its breadth being usually considerable, it is found 

 that a small displacement in one direction or another, makes but a small differ- 

 ence in the effect upon the pile. 



58. The abruptness of the effect of transition fi'om partial to total reflection 

 is far from being so complete as might be wished ; and this is easier accounted for 

 than remedied. It arises mainly from the magnitude of the source of heat, the 



