740 PHILOSOfHlCAL TRANSACTIONS. (^ANNO IJQS. 



is formed round the shadow of eacli, making it less than it should be were there 

 no inflection ; but when the bodies are brought so close to each other that the 

 edge of the one is within the s|)here of the other's inflection, the light being 

 already bent by this last, the former can have none to bend, and consequently 

 no penumbra in the part of the shadow corresponding to that part of the body 

 which is within the other's sphere of inflection; and the rest of the shadow 

 having a penumbra, this part that has none will be larger than it, and increase 

 as the bodies approach, till at last it meets the other shadow; the like appear- 

 ance happening when the shadows are thrown on the eye. Mr. Melvill has en- 

 deavoured to show that it belongs simply to a case of vision; (Edinburgh Lite- 

 rary Essays, vol. 2) however, we have now seen that it has no reference to the 

 structure or position of the eye, but only to the common nature of all shadows. 



Oh. 2. If we shut out all the light coming into a room from external objects, 

 except what may pass through a small hole of -i- or 4- of an inch in diameter, the 

 images of the external objects, as clouds, houses, trees, will be painted on the 

 opposite wall, by the rays of light crossing at the hole: but if a piece of rough 

 glass, or of very fine paper, be held so as to cover it all over, the light does not 

 pass through: then if the paper be wetted with oil, or the glass with water, so 

 as to give either a small degree of transparency, the first rays that come through 

 are those from red and orange objects, and last from blue arid violet. Now it 

 is evident that transparency in general, and this particular fact, are exjjlicable by 

 what was before laid down. It was found by Newton, that a body transmits the 

 light incident on it more or less, according to the continuity of its particles, and 

 that a strong reflection takes place on the confines of a vacuum. How does this 

 happen? The initial velocity of light is sufficient to carry it through the first 

 surface or set of particles, but it is so much diminished, that it is reflected by 

 the repulsive power of the back-side of these ]iarticles, unless there be others 

 behind at a certain distance, namely, that at which inflection or attraction acts, 

 that is, apparent contact ; this attraction renews the impetus of light, and trans- 

 mits it to another set, and so on. Now this action being strongest on the 

 largest and red particles, and weakest on the blue and violet, if the continuity 

 be diminished, the former will be transmitted, and not the latter; which is con- 

 formable to the experiment just now mentioned. 



3. The doctrine of flexibility furnishes an easy and satisfactory explanation of 

 the different colours which are assumed by flame. Whether we suppose the 

 light to come from the burning body, or from the oxygenous gaz, the largest or 

 red particles have the strongest attraction for bodies, the violet the weakest; 

 when therefore the gaz and the body combine, the precipitation of light must 

 be in the reverse order of the affinity between the particles of light and those of 

 the bodies. If then the combination take place slowly, the violet and blue par- 

 ticles will be first emitted, and last of all the red: and this is consistent with 



