VOL. LXXXVI.] PHILOSOPHICAL TKANSACTIONS. 751 



reflexibility of light, I observed, among the regular images made by most of the 

 pins which I used, one or two all of the same colour, as red, blue, &c. and when 

 the pin was moved, these moved also, becoming of other colours in regular order, 

 like the rest ; which shows plainly that their being of one colour at first was 

 owing to some fibre in the surface jutting out, or rather to several of these, 

 which stopped the red and all the rest but the blue of several images, or the 

 blue and all the rest but the red. Further, I produced several regular images 

 by 2 or 3 very small pins, and with considerable trouble I at last contrived to 

 place them in such a position as that one blue colour of considerable size might 

 be produced, then a red, and so on, by altering the posture of the pins ; now, 

 whether the posture or the size be altered it matters not, for the one affects the 

 other. Is it not evident that this experiment, and the conclusion to which it 

 evidently leads, may be transferred to the colours of natural bodies as seen by 

 reflection ? for the parts being specular and spherical, each will form an image of 

 the luminous body ; and by the position of the sides of the neighbouring ones, 

 any 6 of the colours may be stopped, while the 7th emerges; and if this hap- 

 pens in one part, it will happen in all, since that the texture and size of the parts 

 is the same throughout, has never been called in question. Why do many bodies 

 change colours when viewed in different positions.'' Because they reflect 2 colours, 

 or more, of each image to different quarters, and it matters not whether their 

 position with respect to us or our position with respect to them be changed. 

 How do bodies appear 'coloured by transmitted light .'' Because the foregoino' 

 reasonings apply also to the flexion of the rays in their passage through the parts 

 of bodies. These observations appear to furnish a very simple solution of the 

 problem. I shall endeavour, hereafter to confirm them by other remarks and 

 experiments; for it would be superfluous, to illustrate what has been said by 

 figures and demonstrations.* 



Pursuant to these remarks, it will not be difficult to account for the rings of 

 colours of thin plates by reflection, as we before did those of thick plates by 

 flexion ; indeed those formed in the experiment of the two lenses, supposed by- 

 Newton to be owing to the plates of air between them, appear to have a different 

 cause, as may be without much reasoning gathered from the curious experiments 

 of the Abbe Mazeas,-j- and even from one or two of Sir Isaac's own, in which 

 he supposes some medium more subtile than air to he between the glasses. "♦■ 

 I shall now conclude, by a short summary of propositions, containmg the prin- 

 cipal things which have been demonstrated in the course of this paper. 



* It is obvious that the different refrangibility of the rays, will not account for the bright and dis- 

 tinct colours of bodies : if the refracting angle of a prism be continually diminished, till, for ex- 

 ample, it be equal to one of a minute, the refraction will produce no sensible colours ; indeed 

 almost every piece of plane glass has its sides in a small degree inclined to each other, and yet no 

 colours are formed : much less then will refraction tlirough the infinitely smaller parts of bodies 

 produce separation of the rays. 



I Mem. de I'Academie pour I'annce 1738. X Optics, Book 2, Part 1, Obs. 10 and 11.— Orig. 



