452 Dr. Seebeck on the unequal Evolution 



to admit invisible rays, only, ihey say, that our eyes are not 

 sufficiently strong to distinguish them. Should it be the effects 

 perceived beyond the two ends of the spectrum which gave oc- 

 casion to those invisible rays, they would be superfluous on 

 the account alone that our eyes are more susceptible of the et- 

 fects of light than all our thermometers, salts, and leuchtsteine. 

 Thus visible light is found beyond the usual limits of the spec- 

 trum, — it extends even to a considerable distance, and decreases 

 gradually ; and in the same proportion we see its effect on bo- 

 dies decreasing, whether that effect consist in heating, as be- 

 yond the red, or in chemical operation, as beyond the violet. 

 We need not therefore hesitate to consider this light as the 

 active agent in this instance ; for where it is absent, all agency 

 ceases. The assumption of distinct chemical rays proceed- 

 ing from the sun has found few adherents ; that of heating 

 rays has had more. But they must stand and fall together; 

 if we do not allow those of the one kind, the others have no 

 better support. 



I must now add a few words respecting the glimmering light 

 beyond the spectrum. It is not only bad prisms, as M. Wiinsch 

 supposes, but the very best, which have a reflection of light 

 above and below the spectrum. But there are two kinds of glim- 

 mering: one class, with full light and a distinct limit, is some- 

 times occasioned by prisms with streaks and lines in their 

 interior, but is often produced only by an irregular dispersion 

 of light on the edges of the prisms. Such prisms ought to be 

 excluded from this kind of investigation, or the fault be cor- 

 rected by covering the edges. The other kind of glimmering 

 is found with all prisms without exception, even with those 

 that give the limits of the brilliant spectrum in the most di- 

 stinct manner. The glimmering below the red is of a pale red 

 colour ; and that above the violet, of a very pale violet. These 

 weak colours (which become very distinct when concentrated 

 by a lens) decrease as they recede from the main reflection, and 

 ultimately vanish, together with the light, — so that no limit 

 can be given to this glimmering. 



This glimmering was observed by Newton *j but he di- 

 stinctly states that in his measurement he took no notice of it, 

 because he thought that this light proceeded from the clouds, 

 and was irregularly dispersed. In the same manner Herschel, 

 who quotes this passagef, takes no notice of this light or glim- 

 mering, although he found such great effects in it. Nay, even 

 when in the place where his prism excited the greatest heat, 

 he perceived the red colour of this glimmering by means of a 



» Vide 02^ticcs, lib. i. propos. 2, exp. 3. t I'hil- Tians. 1800, j). 319. 



lens, 



