Class of Chemical Bays analogous to the Hays of Dark Heat. 457 



us to insulate a visible red and yellow ray that are without 

 tithonic power, and an invisible tithonic ray beyond the vio- 

 let. 



The solution of litmus enables us to obtain red and green 

 light without action, and an invisible tithonic ray corre- 

 sponding to the violet. 



The solution of bichromate of potash enables us to obtain 

 red and orange light without any tithonic effect. 



Such results might be multiplied without end, for indeed 

 there is scarcely an instance in which spectra of rays that 

 have passed absorbent media are exactly coincident with their 

 corresponding tithonographs. To set the matter plainly be- 

 fore the reader, the following tabular view, gathered from the 

 Plate, may suffice. 



From this, therefore, I infer the entire independence through- 

 out the spectrum of the luminous rays that give to the organs of 

 vision the impression of colour, and the tithonic rays. 



When I come to describe the dark tithonic rays that are 

 analogous to the rays of dark heat, and which are unaccom- 

 panied by any kind of light whatsoever, no further doubt can 

 be entertained on this subject. I have also some other proofs 

 of a very remarkable kind, to be described hereafter, drawn 

 from the phaenomena exhibited by tithonic rays that have un- 

 dergone polarization. 



Next, as to the independence of these rays and the rays of 

 heat. 



One of the most striking proofs of this is the facility with 

 which impressions of the moon's disc may be obtained on Da- 

 guerreotype and other sensitive plates. Even with lenses of 

 comparatively small diameter, and in the space of a few mi- 

 nutes, strong impressions of the moon's surface may be taken. 

 There is no more difficulty in obtaining these sketches than 

 there is in copying a building or a statue, or any other object 

 on which the sun is shining. But the moonbeams have hitherto 

 given no trace of the presence of heat. 



I found, moreover, by direct trial, that plates which had 



