on the terrestrial Rays that occasion Heat. 319 



Newton has taken notice of some imperfect tinges or haziness, 

 on each side of the prismatic spectrum, and mentions that he 

 did not take them into his measures.* 



lgtb Experiment. Refraction of invisible culinary Heat. 



There are some difficulties in this experiment ; but they arise 

 not so much from the nature of this kind of heat, as from our 

 method of obtaining it in a detached state. A red-hot lump of 

 iron, when cooled so far as to be no longer visible, has but a feeble 

 stock of heat remaining, and loses it very fast. A contrivance 

 to renew and keep this heat might certainly be made, and I 

 should indeed have attempted to carry some method or other of 

 this kind into execution, had not the following trials appeared 

 to me sufficiently conclusive to render it unnecessary. Admit- 

 ting, as has been proved in the 15th experiment, that the 

 alternate rising and falling of a thermometer placed in the focus 

 of a lens, when the ball of it is successively exposed to, or 

 screened from, its effects, is owing to the refraction of the lens, 

 and cannot be ascribed to a mere alternate transmission and 

 stoppage of heat, I proceeded as follows .-f My lens, 1,4 focus, 

 and 1,1 diameter, being placed 2,8 inches from the face of the 

 heated cylinder of iron, the thermometer No. 2, in its focus, was 

 alternately guarded by a small pasteboard screen put before it, 

 and exposed to the effects of condensed heat by removing it. 



No. 2. 



Very red-hot. 

 Red-hot. 

 Still red-hot. 

 Still red. 



Newton's Optics, page 23, line 11. f See Plate XVI. Fig. 1 



MDCCC. * T t 



Screened 



o' 



55 



Open 



2 



H\ 



Screened 



4 



5% 



Open 



6 



6of 



