VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 201 



consistent with accuracy and convenience to give a name to this property of light ; 

 we may therefore say, that the rays of light differ in degree of refrangity, reflexity, 

 and flexity, comprehending inflexity and deflexity. From these terms (uncouth as, 

 like all new words, they at first appear) no confusion can arise, if we always re- 

 member that they allude to the degree of distance to which the rays are subject to 

 the action of bodies. I shall only add an illustration of this property, which may 

 tend to convey a clearer idea of its nature. Suppose a magnet to be placed so that 

 it may attract from their course a stream of iron particles, and let this stream pass 

 at such a distance that part of it may not be affected at all ; those particles which 

 are attracted may be conceived to strike on a white body placed beyond the magnet, 

 and to make a mark there of a size proportional to their number. Let now another 

 equal stream considerably adulterated by carbonaceous matter, oxygene, &c. pass by 

 at the same distance, and in the same direction. Part of this will also be attracted, 

 but not so far from its course, nor will an equal number be affected at all ; so that 

 the mark made on the white body will be nearer the direction of the stream, and of 

 less size than that made by the pure iron. It matters not whether all this would 

 actually happen, even allowing we could place the subjects in the situation described; 

 the thing may easily be conceived, and affords a good enough illustration of what 

 happens in the case of light. 



Pursuant to the plan I before followed, I now tried to measure the different de- 

 grees of reflexity, &c. of the different rays ; but though the measurements taken 

 agreed in this, that the red images were much larger than the rest, and the green 

 appeared by them of a middle size, yet they did not agree well enough (from the 

 roughness of the images, and several other causes of error), to authorize us to con- 

 clude with any certainty " that the action of bodies on the rays is in proportion to 

 the relative sizes of these rays." This however will most probably be afterwards 

 found to be the case ; in the mean time there is little doubt that the sizes are the 

 cause of the fact. 



II. Several phenomena are easily explicable on the principles just now laid down. 

 I. If a pin, hair, thread, &c. be held in the rays of the sun refracted through a prism, 

 extending through all the 7 colours, a very singular deception takes place : the 

 body appears of different sizes, being largest in the red and decreasing gradually to- 

 wards the violet. This appearance seemed so extraordinary, that some friends who 

 happened to see it as well as myself, suspected the body must be irregular in its 

 shape. On inverting it however, the same thing took place ; and on turning the 

 prism on its axis, so that the different rays successively fell on the same parts, the 

 visible magnitude of the body varied with the rays that illuminated it. The appear, 

 ance is readily accounted for by the different reflexity of the rays, and follows im- 

 mediately from obs. 2 and 3. 



2. Sir Isaac Newton found that the rings of colours made by thin plates and by 

 thick plates of glass, as he calls them, when formed of homogeneal light, varied in 

 size with the rays that made them, being largest in the most flexible rays. I have 



VOL. XVIII, D D 



