354 



SCIENTIFIC AMUSEMENTS. 



facing one another (fig. 194), so that their centres of curvature and 

 the centres of the mirrors may be in the same horizontal line, 

 supporting the mirrors by any convenient stands; the distance 

 apart may be 3 or 4 yards, or even more. 



Midway between the centre of curvature and the centre of one 

 mirror place a hot object, such as an iron ball, one or two inches in 

 diameter, heated red hot, and on the further side of it place a 

 circular screen a little larger in diameter, so as to cut off all radia- 

 tion from the hot ball towards the further mirror. Notwithstand- 

 ing, heat will reach this further mirror, being that radiated in the 

 opposite direction by the hot ball, and then reflected in a nearly 



parallel beam by the 

 first mirror; the heat 

 thus reflected will be 

 again condensed to a 

 focus by the second 

 mirror, so that if the 

 bulb of a thermometer 

 be placed at the geo- 

 metrical focus of that 

 mirror the mercury will 

 quickly rise. If the 

 mirrors are tolerably 

 accurately shaped, the 



Fig. 194. Two Conjugate Mirrors (reflection 

 of heat). 



heat will be very perceptibly felt by the finger at that point, and 

 a bit of phosphorus or a match may be fired there. This experi- 

 ment, therefore, shows that the same laws govern the reflection 

 from concave mirrors of invisible heat rays as of visible luminous 

 rays. 



Expt. 380. Apparent Eeflection of Cold. Repeat the previous 

 experiment in a moderately warm room (the mirrors not being too 

 far apart), placing a lump of ice in the focus of the first mirror 

 instead of a red-hot metal ball. A thermometer placed in the 

 focus of the second mirror will now indicate a fall of temperature 

 there, apparently indicating that cold is being radiated and re- 

 flected. This effect is due to the circumstance that every body is 

 continually absorbing heat radiated from surrounding objects and 

 losing heat by emission as radiant heat. When the rate of gain by 

 absorption just equals that of loss by emission, the temperature 

 remains constant ; if the former exceed the latter, the temperature 

 rises, and vice versa. In Expt. 379 the thermometer bulb is placed 

 in such circumstances that the total heat received by it (by radia- 

 tion from the hot ball as well as other objects) considerably exceeds 

 that parted with, wherefore an increase of temperature results ; 



