of the Negative or Protecting Rays of the Sun. 91 



point of maximum action. This corresponds with the maxi- 

 mum of protection in the spectrum of daylight. 



The white stain of the spectrum of darkness is apparently 

 narrower than the protected space in the spectrum of daylight. 



Rays of luminous or of non-luminous heat projected on the 

 darkness or daylight spectra during their formation appear to 

 exert no kind of special influence on the result. 



The white fringe which borders the solarized portion is not 

 due to anything analogous to conduction. These chemical 

 changes, unlike thermal changes, cannot be conducted. 



By interposing between the prism and the Daguerreotype 

 plate a convex lens of short focus, so as to intercept in suc- 

 cession each of the coloured rays, I threw all over the plate, 

 while the spectrum was in the act of being impressed upon it, 

 red, orange, yellow, &c. lights in succession; the object being 

 to ascertain how far the impressed spectrum would change 

 when these monochromatic rays were used along with day- 

 light. Sir J. Herschel having previously showed in similar 

 experiments that new phsenomena arise during the conjoint 

 action of rays*. The following are some of the observations 

 I made; their date is Sept. 24, 1812. 



The red ray when projected increases the length of the so- 

 larized portion, and also of its white extremities. 



The yellow ray shortens the solarized portion. 



The green ray exerts a greater action of the same kind. 



The indigo ray gives a most remarkable result. It totally 

 inverts the action of the less refrangible rays ; and they solar- 

 ize the plate, acting precisely in the same way that the more 

 refrangible rays commonly do, causing it to exhibit a watch- 

 spring lustre. 



I further found that when different rays are brought to act 

 upon each other, the result does not alone depend upon their 

 intrinsic differences, but also on their relative intensities. Thus 

 the green and lower half of the blue rays, when of a certain 

 intensity, protect the plate from the action of the daylight ; 

 but if of a less intensity, they aid the daylight. 



The red and orange rays, when of a certain intensity, in- 

 crease the action of daylight on the plate ; but if of a less in- 

 tensity, they restrain it. 



These facts seem to be connected with the circumstance 

 that there is often to be traced on Daguerreotype plates a 

 remarkable difference between the central and lateral parts of 

 a spectrum. Thus if a line be drawn through the centre of 

 sue!) a spectrum and a parallel to it on one ol" the edges, the 

 action at any point on the central line is the reverse of that at 

 • Phil. Trans. 1840, § G4. 

 H2 



