Terrefirial Rays thai cccajion Heat. a$g 



catlnot be accurately afcertained by looking at it in a room 

 hot Sufficiently dark to make very faint tinges of colour vi- 

 fible. And to this rauft be added that the incipient red 

 rays rauft actually be fcattered over a considerable Space, 

 near the confines of the Spectrum, on account of the breadth 

 of the pnfm, the whole of which cannot bring its rays of 

 anv one colour properly together ; nor can it feparate the in- 

 visible rays entirely from the vifible ones. For, as the red 

 rays will be but faintly fcattered in the beginning of the vi- 

 fible Spectrum ; fo, on the other hand, will the invifible raySj 

 feparated by the parts of the prifm that come next in SucceS- 

 fion, be mixed with the former red ones. Sir Ifaac Newton 

 has taken notice of fome imperfe£t tinges or hazinefs on each 

 fide of the priSmatic Speftrum, and mentions that he did not 

 take them into his meaSures*. 



igtb Experiment. RcfraEtion of 'invifible Culinary Heat* 



. There are fome difficulties in this experiment; but they 

 arife'not fo much from the nature of this kind of heat, as 

 from our method of obtaining it in a detached ftate. A red- 

 hot lump of iron, when cooled fo far as to be rto longe r 

 vifible, has but a feeble Stock of heat remaining, and lofes 

 it very faft. A contrivance to renew and keep this heat 

 might certainly be made, and I flioukl, indeed, have at- 

 tempted to carry fome method or other of this kind into exe- 

 cution, had not the following trials appeared to me Suffi- 

 ciently conclusive to render it unneceffary. Admitting, as has 

 been proved in the 15th experiment, that the alternate rifing 

 and falling of a thermometer placed in the focus of a lens, 

 when the ball of it is Successively expofed to, or fcreened 

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

 cannot be afcribed to a mere alternate tranfmiffion and Stop- 

 page of heat, I proceeded as follows f : — My 1 :as, 1-4 focus, 

 and i'i diameter, being placed vS inches from the face of 

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

 focus, was alternately guarded by a Imall paiteboard fcreen 

 put before it, and expofed to the effects of eondenfed heat by 

 removing it. 



Newton's Optic;, p. t : , I. 11. + See Plate VIIl. fig. 1, 



L 1 Z No. 



