VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 703 



the margin of the pasteboard, I let the invisible rays beyond the spectrum fall on 

 the lens. In the focus of the red rays, or a very little beyond it, I had placed the 

 ball of the thermometer N° 1 ; and, as near to it 



as convenient, the small one N° 2. Now, that the Min> In the Fo * cus . Near ^ j^ 

 invisible solar rays which occasion heat were ac- 57 57 



curately refracted to a focus, may be seen by the 



annexed account of the thermometers. Here, in l m , these rays gave 45° of heat 

 to the thermometer N° 1, which received them in the focus, while the other, 

 N° 2, suffered no change. It is remarkable, that notwithstanding I kept the red 

 colour of the spectrum t l- of an inch on the pasteboard, a little of that colour 

 might still be seen on the ball of the thermometer. This occasioned a surmise, 

 that possibly the invisible rays of the sun might become visible, if they were pro- 

 perly condensed; I therefore put this to the trial, as follows. 



Exper. 1 8. Trial to render the Invisible Rays of the Sun Visible by Condensation. — 

 Leaving the arrangement of the apparatus as in the last experiment, I withdrew 

 the lens, till the last visible red colour was -fa of an inch from M . N<3 

 the margin of the semi-circular pasteboard cover; then, taking o 57 57 

 the thermometers, I had as annexed. Here, there was no l 78 57 

 longer the least tinge of any colour, or vestige of light, to be seen on the ball of 

 the thermometer; so that, in l m , it received 21° of heat, from rays that neither 

 were visible before, nor could be rendered so by condensation. 



To account for the colour which may be seen in the focus, when the last visible 

 red colour is less than fa of an inch from the margin of the pasteboard which in- 

 tercepts the prismatic spectrum, we may suppose, that the imperfect refraction of 

 a burning lens, which from its great diameter cannot bring rays to a geometrical 

 focus, will bring some scattered ones to it, which ought not to come there. We 

 may also admit, that the termination of a prismatic spectrum cannot be accurately 

 ascertained, by looking at it in a room not sufficiently dark to make very faint 

 tinges of colour visible. And, to this must be added, that the incipient red rays 

 must actually be scattered over a considerable space, near the confines of the 

 spectrum, on account of the breadth of the prism, the whole of which cannot 

 bring its rays of any one colour properly together; nor can it separate the invisible 

 rays entirely from the visible ones. For, as the red rays will be but faintly scattered 

 in the beginning of the visible spectrum, so on the other hand will the invisible 

 rays, separated by the parts of the prism that come next in succession, be mixed 

 with the former red ones. Sir Isaac 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. Opt. p. 23, 1. II. 



Qocper. 1 9. 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 



