66 



Prof. G. G. Stokes. 



[May 25, 



with the diffused light from all parts of the sky, they did more harm 

 than good. They behaved, in fact, like the rays from the lamp in the 

 experiment of § 6. The slightly inferior luminosity of the parts to 

 some little distance on "both sides of that on which the shadow fell, 

 shows that the loss of the diffuse light corresponding to the portion 

 of the sky cut off by the pole was quite sensible when that portion lay 

 very near the sun. 



All this falls in very well with what we know of the nature of the 

 direct sunlight and the light from the sky. In passing through the 

 atmosphere, the direct rays of the sun get obstructed by very minute 

 particles of dust, globules of water forming a haze too tenuous to be 

 noticed, &c. The veil is virtually coarser for blue than for red light, 

 so that in the unimpeded light the proportion of the rays of low to 

 those of high refrangibility goes on continually increasing, the effect 

 by the time the rays reach the earth increasing as the sun gets lower, 

 and has accordingly a greater stretch of air to get through. Of the 

 light falling upon the obstructing particles, a portion might be absorbed 

 in the case of particles of very opaque substances, but usually there 

 would be little loss this way, and the greater part would be diffused 

 by reflection and diffraction. This diffused light, in which there is a 

 predominance of the rays of higher refrangibility, would naturally be 

 strongest in directions not very far from that of the direct light ; and 

 the loss accordingly of a portion of it where it is strongest, in conse- 

 quence of interception by the pole in front of the tablet, accounts for 

 the fact that the borders of the place of the shadow were seen to be a 

 little less luminous than the parts at a distance. 



8. The observations on phosphorescence just described have now 

 prepared the way for the explanation I have to suggest of the photo- 

 graphic phenomenon. 



It is known that, with certain preparations, if a plate be exposed 

 for a very short time to diffuse daylight, and be then exposed to a 

 pure spectrum in a dark room, on subsequently developing the image 

 it is found that while the more refrangible rays have acted positively, 

 that is, in the manner of light in general, a certain portion of the less 

 refrangible have acted in an opposite way, having undone the action 

 of the diffuse daylight to which the plate was exposed in the first 

 instance. 



It appears then that in photography, as in phosphorescence, there 

 may in certain cases be an antagonistic action between the more and 

 less refrangible rays, so that it stands to reason that the withdrawal 

 of the latter might promote the effect of the former. 



Now the objective of a photographic camera is ordinarily chemically 

 corrected; that is to say, the minimum focal length is made to lie, 

 not in the brightest part of the spectrum, as in a telescope, but in the 

 part which has strongest chemical action. What this is, depends 



