of Heat in the Prismatic Spectrum. 333 



$ition, into the individual colours of the prism, and kept in 

 each until the liquid became stationary. The light under ex- 

 amination only fell on the bulb. A moveable screen, fixed at 

 a distance of from 12 to 18 inches from the thermoscope, in- 

 tercepted the remaining pi'ismatic colours. The screen was 

 so made that the opening through which the light fell could 

 be enlarged or contracted ad libitum. Sometimes only half 

 the screen was employed, at others it was removed altogether ; 

 but this is mentioned every time it was done. 



The room in which the experiments were, made (mostly 

 between 11 and 12 o'clock) lay facing the south, was large, 

 and could be completely darkened. The prism was some- 

 times fastened in the window-shutter, and no other light was 

 admitted into the room Ijut that which passed through it. 

 But in most cases I let the light fall through an opening in the 

 shutter of about 49 square inches on the prism, placed on a 

 moveable stand on the window-board. The refracting angle 

 of the prism was always directed downwards, its whole side 

 surface being left free, and only the upper third surface co- 

 vered with black paper ; which was also done sometimes with 

 the lower angle, in order to obtain the coloured image more 

 strictly confined, and to keep off all improperly reflected light. 

 The prism had at the beginning of the expei'iments such a 

 position, that the two angles in which the light entered and 

 passed out were equal, which position I will call, for the sake 

 of brevity, the normal position. In order to throw the colours 

 on the thermoscope, the prism was afterwards sometimes 

 brought into a different position by being turned on its axis ; 

 but for the most part I left the prism unchanged, and brought 

 the bulb of the thermoscope, by raising or lowering it, into the 

 different prismatic colours. That this latter process could be 

 adopted without prejudice, (at least in cases when the thermo- 

 scope was carried down from the cold into the warmer colours,) 

 I was convinced by experiments made on the temperature of 

 the room at different heights, which showed that the air to- 

 wards the floor (which consisted of clay) was always a little 

 colder. If therefore the warmtli increased whilst the thenno- 

 scope was lowered into the prismatic spectrum, the experi- 

 ment was thereby more strongly confirmed. By way of 

 greater security, however, a second air-thermometer was some- 

 times placed by the side of the spectrum, of which instances 

 will be mentioned. 



The glass prisms whose effect was tried were of different 

 qualities and sizes, varying from three-fourths of an inch to 

 one inch and a (juarter in breadth of surface. They also dif- 

 fered in the refracting angle, although not considerably, as it 



approached 



